Start with one, but... once they do one hour they are most likely going to want to do two. It's addicting!
2 hours would be better.
1 hour is only about enough time to get 2 or possibly 3 rounds in. When we went, we found a Groupon for an all day pass with equipment rental at ~$20 and we're essentially limited to however much we wanted to spend on paintballs. I would definitely suggest getting them by the large bags, it's a much better deal and you'll go through them quickly. Have fun!
At least 2 hours if not more because games are long and if you get out early you're just waiting for the next game.
I would do 1 hour first
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