Technically there is no formal time limit on how long an applicant can be abroad while the application is in process.
Those who applied after requirements changed in June 2015 are required to intend to continue to reside in Canada. Many references to this requirement overlook or omit the continue to reside aspect of the requirement. Logically a person cannot intend to continue to do something unless the person is already, presently doing that thing. Thus, technically, just residing abroad means the person cannot have an intent to continue to reside in Canada, since that person is not even residing in Canada.
But it appears IRCC, under Liberal leadership, might not be enforcing this requirement beyond it still being an affirmative declaration the applicant must make in the application itself. The proposed changes in Bill C-6, if adopted, will not only remove this requirement but will result in it being deemed there never was such a requirement.
All that said: extended absences from Canada, while the application is pending, have caused problems. There are a number of topics in this forum which discuss this in more depth. Thus, while there is no disqualification or other limitation on how much time the applicant can be abroad after applying (other than, as noted above, the requirement to remain in compliance with the PR Residency Obligation, and the "intent" issue), many of those who are abroad for an extended period of time have been subject to RQ, delays in processing, elevated scrutiny, in addition to the potential logistical problems which can arise due to short notice for scheduled appearances. See some of the other topics for more in-depth discussion of the problems some face.