10w at 12v is approx 10/12 = 0.83A , so a typical 60AH battery would last 72 hours. In reality you'll get less than that due to it not being 100% efficient and the battery dropping off towards the end of it's useful power.
Even with LED or CFL lighting I'd think you'd need a lot more than 10w to light a garage to work in. eg. I have 3 x 23W CFL's (equivalent to 3 x 100w of "normal" bulbs), that's nearly 70W and I still need a separate work light for close up stuff.
Obviously depends on if you're working during the day and the garage has windows etc.