I agree with what girlofgod said. I still have a betta in a 2.6 litre vase, and he is well over 3 years old. At one point I put him in my 25 gallon aquarium with black skirt tetras and some neons. He did not bother the other fish, and they left him alone, but he didn't look 'happy' in there; he tended to hide in the bottom of the tank, under a log with the pleco. So I put him back in his little container by himself, and he is still going strong.
You know what the definition of hobby is: something of interest that is going to cost you lots of money!