I did this once too. But I left it for a couple of weeks.

It was when I got home from a rally, and I had to go to work the next day. So I just rode in and put her up and didn't think about it again.
I've got my bike in an enclosed cover so I didn't notice anything was amiss until I smelled gas just before I was going to get it ready for a ride the next day. There was a puddle of gas under the bike, so I knew I'd left the petcock open (again). I didn't think much about it and turned the petcock off. Then I checked the oil................. hmmmm, this is a little thin..................and smells an awful lot like gas.

I'm glad I checked it before I started her up. I didn't have to worry about pumping gas throughout the whole system. I ended up dumping all that gas/oil out and changing out the filter and putting in all fresh oil. No problems. (whew)
Ed Miller wrote on Today at 8:47am:
Well, and fix the carbs so they don't flood. That shoudn't happen even with the petcock open. New needles and/or floats, or check the level.
I have a comment on this. These carbs are not designed to hold back the flow of gas. That's what the petcock is there for. This is even stated on the website. But, saying that, the carbs should be adjusted/rebuilt so they work at their best just to make sure.