21 days without a login from a studio boss, triggers a vote to find a new boss. So I take that as precedent that the game considers you gone after 21 days.
105 days with no login (plus or minus a little, we think) causes an avatar to go poof, and auto-delete.
I hate the fact that from day 22 to day 105, zombie avatars get to keep their full skill boost. That is a long time for an avatar to be powered up, but not paying for that boost.
I suggest that after 22 days of not logging in, personal boosts should reset to nothing.