I’d say 15 to 20 hours. I forget the calculations, but I think with the pump I use , I should be able to get something like 38 hours from it, but it takes no time to pull it off truck, don’t want to push it , and put it on a charger overnight. Are you using a Marine battery or regular car battery? A car battery really isn’t designed to use with pumps and gets used up pretty quick.
I installed a deep cycle marine battery as the starting battery for our car a couple years ago. This really isn’t recommended, but it’s worked well for our needs. The battery is a bit oversized, but fits snuggly where our old battery went. The battery just charges off of the alternator, and the amount of travel involved in our service area has kept the battery charged well. I’m also not using the WFP every day I work. But I can do a full day of WFP work and not have to worry whether the car will start.
I wired the power cord for the pump into the back of the accessory outlet, so I can still plug in phone chargers and the like.
Marine (deep cycle) batteries are designed to be discharged completely or nearly so. Are best charged and maintained with a trickle type charger. You can usually get one at a box store or automotive store cheap enough. They even have ones that mount them where you want them, and just plug it in when you need to charge your battery. Fast charging a deep cycle is not good. So it has never been a good idea to mount one to start a vehicle (high output charging systems) or to fast charge with a charger.
Car batteries are NOT meant to be discharged like a deep cycle. It can kill the cells in the battery. Besides, it is what starts your vehicle. Why mess with it? A tow bill or calling for a jump will usually cost you more than a deep cycle battery.