Three days to a week would be fine. I don't expect something like that to last forever.

I would prefer not to confront him directly by making him grovel -- I mean, he's in Dark Arts and he's two years ahead of me. He'll KNOW who and why, anyway, but I'd rather he didn't realise until later that I've hexed him. Especially late at night, after curfew, because he won't be able to do a thing about it until the next day, even go to Madam Pomfrey (and even if he goes straight to her for a cure it'll be utterly humiliating. Who wants to admit to the matron that they wank? Even though it's not as if she doesn't KNOW.)