This is a unit of measurement for battery capacity, obtained by multiplying a current flow in amperes by the time in hours of discharge. (Example: A battery which delivers 5 amperes for 20 hours, 5 amperes times 20 hours, or 100 ampere-hours.)
Very simply if you have (2) 500W motors at 24V that is about 42Amps max draw (1000/24) If you have (2) 12V 35Ah SLA batteries in series (24V at 35Ah) then 35Ah/42 = 0.8h or 50min of run time.
Alternately, if you have (1) 500W motor at 24V that is about 21Amps max draw (500/24) If you have (2) 12V 18Ah SLA batteries in series (24V at 18Ah) then 18Ah/24 = 0.7h or 45min of run time. (Note that 500W is quite under-powered, since we can go as high as 1440W in a PPPRS vehicle.)
The numbers presented above are in a perfect world with no losses. As the motors get hot and the batteries get hot there are losses so the actual run time would be less.