You took a decent amount. Not much would have been something like 1-2mg per day, so 3-6 mg total for 3 days. Taking 18mg over the course of 3 days has definitely put some blockade on your receptor. I would wait at least 48 hours, preferable 3 days after taking that much. Bupe has a long half life and a high affinity for your receptors so it stays around for a while. After 24 hours you will feel high but the effects would be greatly diminished. After 48 it will be much better, but likely not fully what you could get for you money and after 3 days I think you will be in the safe and will get what you paid for with the roxies.
You are basically looking for the bupe to totally leave your system. Any left over bupe will compete with the roxies at the receptor and win because bupe has a higher affinity than oxycodone. Bupe has a very long half life so it stay around for a long time. Best to wait as long as you can. Bupe has an average half life of 37 hours...so after 37 hours the average person still has half the bupe left in their system. that bupe will compete with the oxy and win. And that doesn't mean it takes your body 37 hours to get rid of half the bupe...it might take your body 60 hours. So just to be safe I would wait 3 days at least.