I guess it depends on the mistake the person did.
Normally, I tend to forgive people after a few hours, or minutes, if it wasn't a bad mistake.
But when people really get on my nerves and make a mistake that REALLY bothers me, I'd say that I need to get distant and avoid the person for at least a day. Then, probably, I'll forgive the person and everything is fine again. ^^"
All in all, I forgive EVERY mistake a person makes. Sometimes, though, I change my opinion about a person.