36 two-week periods, about 72 weeks (~15 months) to get back 500 activity
unless the system has inner counter keeping track maximum potential activity even with deleted posts
if he's a bit smarter, he would have left 1 post in every two-week period to lock in potential activity
They definitely lose the activity permanently. It is odd they deleted the posts after getting tagged as the account is worthless now anyway. Maybe they saved all the posts and want to reuse them from another alt account they have without getting caught for plagiarism.