Thanks for the update @Douwe! 
Maybe you don’t have to. At least, I cannot remember not being able to edit a post and I tried it right now: I’m able to edit my posts from Jan 31st, but not those from Jan 29th. So I guess, my time limit is 2 months.
@Douwe: Does this limit depend on the user access level? None of the bullets in the topic Forum User Access Levels Explained mentions this.
That seems plausible as long as wiki posts are not affected (and in fact, they seem to be excluded).