On Walmart.com today, I see a Quarrow 36-LED in that price range. I ended up purchasing the 90-LED one.
When I was selecting a light and battery combo, I made a conservative estimate of 60% utilization, because batteries don't yield full output when cold and I don't want to drain a battery to zero. I was shooting for 8 hours of service. I called the light manufacturer and they kindly gave me the current draw for each model at the time. Like willie said, it's easy math. Here is the chart I used to select my 90-LED light and an 8ah battery:
I have run it 7-8 hours and never run the battery down.
Hope this helps.
IMO
YMMV