It's currently set around 70 days, if I'm not mistaking. This will be changed in the new site, where new users are also no longer able to 'necromance' old threads.

I want new users to have somewhere around a week editing ability and established members a limit of 6 months or something along those lines. Then there's the "special" members who maintain very helpful or cool topics, like the Pinball Quiz topic by Soren or the guides by Vid. These need constant updating and therefore these Pinsiders get unlimited editing rights (the latter has already been implemented).