If you've bought before then it's usually almost instant, if it's your first time buying for that character then I think it involves some manual authorisation, so the time can vary a bit on how busy GM is, sometimes can be quick in a few minutes, sometimes (rare occasions) I've heard of it being the next day.
It uses paypal so it's very unlikely to be down or offline unless he has an account issue, in which case I don't think it would let you make the payment at all
Sorry it's not a perfect answer*, as I haven't bought any today, but I hope it's at least a bit helpful
* disclaimer required as some people in the past have got really angry at me for only being a little bit helpful instead of giving a complete answer