My cousin is coming to Canada as a live in caregiver. She has been given a visa and a Letter of Introduction. Her current employer (outside of Canada) won't allow her to leave yet. Is there a time limit in terms of coming in to Canada and get her work permit? If she does not come to get her work permit in 3 months, will she have a problem upon entry to Canada?

Thanks for any advice you can give.

At a minimum, arrival within the validity of the entry visa is needed. The earlier she arrives in Canada to collect the work permit, the less the chance that an immigration official at the port of entry would question the continued validity of the work permit approval.