If you only used it once, then I would wait at least 24 hours and after that you should be able to get a decent rush, but not as strong if you were to wait 36-48 hours. But I use to take 4mg of bupe once in a blue moon, and I would always wait 24 hours and then would be able to get a good, normal rush. But that is if you took no more than 4mg and you haven't been taking it daily.
If you have been taking it daily then it will take at least 48-72 hours to get a half-assed rush, and probably close to 5 days before it is at full strength again. But if I were you I would at least wait at least until tomorrow morning, preferably in the evening since you took 8mg. Trust me, I have waisted plenty of D's by being an impatient bastard, but I do realise how hard it is to wait to shoot up when that dope is sitting their calling your name lol.