I've done some research on this topic. There is no requirement in the regulations/Ministerial Instructions that the applicant should have performed at least 50%/60% or whatever percentage of the main duties stated in the NOC.
However, my advice would be to try to match as many of the duties as possible just in order to be on the safe side, as it seems that the process is very bureaucratic and there is no guarantee that you won't come across an officer who is unreasonable or incompetent.
But, as some people have pointed out, it is close to impossible for some occupations to match most of the duties in the NOC. Also, there are NOCs with only 4-5 duties, while others have 12-15. Some NOCs also lump together the duties of several separate positions - for example NOC 1123, which has 16 duties.
This is what the Ministerial Instructions say:
(b) must have performed the actions described in the lead statement for the occupation as set out in the occupational descriptions of the National Occupational Classification, as well as a substantial number of the main duties, including all essential duties, as set out in the National Occupational Classification.
http://www.cic.gc.ca/english/department/mi/express-entry.asp
There is no definition of what "a substantial number" means, but it does not unequivocally mean "most". In fact, there is a case in which the officer rejected an application on the grounds that the applicant did not perform more than 50% of the NOC duties. The applicant filed for judicial review, and the court ruled that the officer was in error.
Here is a link to the case: http://decisions.fct-cf.gc.ca/fc-cf/decisions/en/item/40034/index.do
Here is an excerpt from the decision at the link above:
The officer erred in law by requiring the applicant to have performed a majority of the duties listed in the NOC to be assessed units for experience and occupation as a civil engineer.
[14] Thirteen "main duties" are listed are in the NOC for civil engineers. However, the NOC states that "civil engineers perform some or all of the following duties". This explains that civil engineers do not necessarily perform every duty listed. The NOC specifically states that civil engineers perform "some or all of the duties". "Some" is defined in the Concise Oxford Dictionary (8th Edition) as "an unspecified amount or number", "a considerable amount or number", and "at least a small amount".
[15] The language in Item 4(1)(b) of Schedule I contains general language which applies to all NOC occupations. This general language is modified by the specific language of the NOC with respect to civil engineers. The Latin maxim of statutory interpretation generalibus specialia derogant (special things derogate from general things) applies. The NOC description of a civil engineer specifically states that "civil engineers perform some or all of the following duties:", and then lists thirteen duties. This specific language qualifies the more general language so that "some or all" takes precedence and supersedes the general language "a substantial number of the main duties as set out in the NOC". "Some" means more than one.
Note that the judge ruled that since the phrase "some or all" in the NOC description is more specific than "a substantial number", then "some or all" takes precedence.
Here is a similar case: http://www.canadavisa.com/case-commentary-2013.html
February 22, 2013: Janna Malika Benoit vs. Canada (Minister of Citizenship and Immigration)
The applicant submitted an application under Canadian Experience Class through her experience under NOC 6211. This application was rejected as the visa officer believed that the duties listed in the reference letter submitted by the applicant did not match the duties listed in the NOC description. As per the regulations, an applicant must perform a substantial amount of the main duties, as well as all of the essential duties of the NOC in question. However, NOC 6211 does not have any essential duties. Therefore the requirement is only that the applicant performs a substantial number of the main duties.
The officer mentioned in their notes that “ordering” and “scheduling” were done by the applicant only under the supervision of the manager. However, these activities are only components of the main duties listed in NOC 6211. It is not clear if the officer attempted at any point to determine whether or not the duties were a substantial match overall. While the actual assessment must be completed by a visa officer, the court is satisfied that the applicant at least performed some of the duties in her selected NOC, and therefore her application for permanent residency has some chance of success. Therefore, the application for judicial review is allowed. The application will be remitted back for decision by a different officer.
However, keep in mind that in some cases the officer is entitled to give greater weight to some duties over others, as this case demonstrates: http://caselaw.canada.globe24h.com/0/0/federal/federal-court-of-canada/2000/05/26/farooqui-v-canada-minister-of-citizenship-and-immigration-2000-15363-fc.shtml
Here are some more similar cases:
http://visalawcanada.blogspot.bg/2014/01/applicant-did-not-perform-duties-in.html
http://www.lexology.com/library/detail.aspx?g=cffbc835-dcff-4c37-9692-5757c247edab
http://www.lexology.com/library/detail.aspx?g=83e5e75f-1766-4bce-9aa4-81afd55d704a
http://visalawcanada.blogspot.bg/2013/08/applicant-job-duties-at-issue-in.html
Some of the cases presented above were rejected by the court, but they had other issues that caused them to be denied. But it is evident that the court thinks that the minimum standard when evaluating duties against the NOC should be that the applicant should have performed "some or all" duties, i.e. at least two, and not "most".
But I would like to reiterate that performing at least two duties does not mean that you can claim that you belong to this NOC, especially if these duties are generic and are part of many other NOC codes. Remember: you have to meet the lead statement AND some or all of the main duties. So even if you did meet most duties but not the lead statement, you can be rejected, as it seems that the lead statement is considered to capture the essence of the occupation. Btw, I've noticed that the first duty in many NOC codes is usually some variation of the lead statement.
And again, if you can, make sure that you meet at least 50% of the main duties, although it is not required in the Ministerial Instructions. There are officers who are clearly incompetent or make mistakes, which is to be expected, so it's better to be safe than sorry. To me it is quite concerning that there are still visa officers who state that you have to have performed at least 50-60% of the main duties given that there are court cases where such a requirement by the officer has been rejected as erroneous.
If you are rejected because you didn't match a substantial number of the duties in the NOC, I suggest you talk to a lawyer as soon as possible and file for judicial review if your lawyer thinks you have a case. As it can be seen from the court decisions presented above, some people do succeed in getting their application reconsidered by a different officer in case the VO acted unreasonably.