I think it varies on quite a lot of things. Most people I know have very active email lives, so for most things I'd assume the default is "Yes, that's fine. But I'll probably forget unless you poke me by email". I'd start to count pestering from the second email, because I can, and have, spent years intending to give something to someone, getting in person reminders, and forgetting again, or vice versa.

And it depends how big a favour it is -- if it's small, you feel dafter about pestering, but more annoyed that they forgot, whereas if it's larger, or more time dependant, you don't want to pester, but don't want to let it go either.