Don't worry too much, sometimes there are surprises from CIC too.
We filed for parents back in 2009, and sponsorship got approved in Jan. 2013, and by Sept 2013 we had visa issued and parents landed too.
Our file went to Paris office, their timeline for parents was over 40 months, but we got finalized within few months. We had 3 teenage dependents too on file so not bad in the end.
Some offices are slow like Singapore, Islamabad, etc but some are relatively fast. But eventually all will be decided. Get parents to keep their health in good, by daily walk, care of diet, etc so that when medical time comes, it goes well.
When our file was at CPC-M, it seemed like everyone's case was being approved before ours and it seemed as if we were in the slowest queue, but it picked up good pace at the visa office. Now their PR Cards are already 1.5 yrs old!
A lot also depends on how organized and easy your file is. We had also included a detailed letter on pretty much every document and person in the file. Remember CIC wants everyone's history since age 18(not the last 10 yrs even if the guide says so, so make sure personal history and activities are dated from since age 18 - if it is hard to remember addresses and dates, use approximate and indicate that in a letter with signatures). No gaps, include everything since age 18. Try to get the PCC's done as well since in some countries it takes time, and also a lot of fees/money.