True, hadn't considered the farm account type, [@Mahz]. But only a fraction of spambots are of that type, yes? 

So maybe a combination would work?
If a user starts posting immediately, it'll remove limits by the 72[sup]nd[/sup] hour.
If a user doesn't post in that time, it will not remove the limits until a certain post count has come and gone.

That way, people can get rid of limits in multiple ways, even as multiple spambot types can be stopped. After all, once a user starts posting, it'll be fairly easy to spot bots.

Granted, the above might be overly complicated.