How long is a piece of string?
How big is the bottle? Does it have an anti-syphon tube?What marker do you use? Does it have an expansion chamber or in-line regulator? What's the temprature?The altitude? How much paint do you use?
All of this affects how long the gas will last. Generally, a 12 oz tank lasts about 600 rounds but it varies a great deal
I wouldn't advise using a paintball marker anywhere other than at a paintball field. Anywhere else is just not safe. Also, using a marker outside might draw unwanted police attention. Most people would just see a gun.