Without getting all technical, a 100W bulb will give you roughly 35 minutes of good light on that battery. Usually the SLA heats up during discharge which further lowers the voltage. You'd probably still have useful light for 45+ minutes but it would appear fairly orange. You'd need at least six of those batteries to run that light for 6 hours continuously and the performance on back half of each hour would be generally pretty sad.
You could run two 10Ah li-ion packs in parallel but your still looking at about 4-5 lbs as that's 24 X 18650's cells plus wiring, connectors and wrap. Additionally the price might be more than you're willing to pay for such a set-up as you could easily spend over $300. Even at that it's still only 20ah of power and not nearly enough for 6 hours of continously use which would require a full size automotive 12V deep cycle battery, and you know what those weigh!
6 hours of constant run sounds like the perfect application for HID. If you can at all get away with it I'd even lean toward LED but since your using a halogen spot for whatever you're doing I have doubts that LED would give you acceptable performance.
I'd look as some of the existing HID systems from Trailtech.
http://www.batteryspace.com/index.asp?PageAction=PRODSEARCH&txtSearch=trailtech&x=0&y=0
This system will provide 4 hours of continous use at 26W
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2883
The final alternative is to run far less output than you're considering doing now. A 15 or 20W bulb will run for 6 hours off that battery which probably means a different model of incan light alltogether.
