I have one bike with a 55 watt headlight and I too run a 12v 7.5ah battery. I've gotten a good solid 90 minutes of usable light. Along with the big headlight I also have a brake and tail light running from the same battery I typically never ride that long at night, usually less than a half hour so I'm satisfied with the battery life.
Of course, a smaller wattage lamp will lengthen the time so I'd say you're good to go.
Tom