Battery life

[Bilbo]

I just got a new Revvie. It takes 2 9V batteries. Can anyone tell me about how long I can expect two batteries to last? Will I make it through a day of heavy firing?
I am talking about disposable batteries here, not rechargeables. My guess is that rechargeable NiMH would likely last longer, but I only have one and I use that in the marker.