For a lead–acid battery
is typically between 1.1 and 1.3. For different lead-acid rechargeable battery technologies it generally ranges from 1.05 to 1.15 for VRSLAB AGM batteries, from 1.1 to 1.25 for gel, and from 1.2 to 1.6 for flooded batteries.[2] The Peukert constant varies with the age of the battery, generally increasing (getting worse) with age.
I recon you would get around an hour, maybe less. Also if your batteries are standard car (auto) types, the will not last long. You need deep discharge types as designed for boats, campers etc.