In most areas of a national forest you're limited to camping for 14 to 21 days depending on the area you're camping in.
But most areas of national forests allow you to camp for up to 14 days in the same area and then you must move your RV, Travel Trailer or tent or whatever you're camping in to another spot.
Arizona and California have areas in their national forests that you can camp for months at a time without needing to move and those areas should have signs showing the amount of time you're allowed to camp in the area.
The other parts of the national forests also should have signs showing the time limit to camp in the national forests but mostly the limit is 14 to 21 days.