Most are 1 hour, some are 2 hour... Depends on the building type, and local adopted code, engineering, etc.
Most sets of plans that get stapmped by the city here will have wall and ceiling schedules, and paramiters laid out on a detail page. Like 'no openings in envelope over 16 sq inches' (7sq" in some places?), 100" of opening in 100'sq feet of wall', etc.
Here's a nice article by UL about boxes in walls, that I used for a class with my old crew... Most of this stuff is rule of thumb.
http://www.ul.com/regulators/sepoct2000.pdf
