I leave gas in my mower all through the winter and it starts up pretty easily in the summer. I did this for about 3 years straight and after 3 years it was running kind of rough. I poured in some Sea Foam with the gas and after a few minutes that damn mower ran really nicely. It starts up on the first pull of the string now too.
I know it isn't a motorcycle but just saying the gas should be ok for a while in there but if you need to clean up the fuel system I can vouch for that Sea Foam stuff.