Do you mean how long it will run continuously until the water/meth runs out?
For most of us, the spray only comes on in short bursts. The Snow Performance kit allows you to set the point where it comes on, either at a specific boost pressure or rpm level. Its up to you.
I understand your needs may differ from most enthusiasts and I can't really answer your question. I would call Snow. I have called them with technical questions before and they have engineers that will talk to you.
Snow also sells a remote reservior you can place in your trunk. You may even want to fab up a larger tank to hold more fluid.
gallons per hour will depend upon nozzle size: they are rated for volumetric flow so the answer is probably closer to you than you think. The nozzle sizes are:
60 ml/min, 100 ml/min, 175 ml/min, 225 ml/min, 375 ml/min, and 625 ml/min. My guess is that you would run dual 225ml nozzles or some mix thereof that will flow near 500ml/min at max flow.
Snow makes a 2.5 gallon tank for the trunk, and that is what I have in my mustang...