Using Map Pilot Pro without internet

There is no limit to how long you can be offline in the app. As long as you go offline with an account that has an expiration date that is further out than the period you intend to use it. 

There MAY be limited to how long the Apple Maps basemap gets cached. We have tested out to a few weeks of being offline but never 3 months. As long as you have your missions saved with basemap and terrain information for the areas you intend to be working it that data will be available.