The most common answer is "as much as you can give them". In my 3 years of keeping a sulcata I have found this to be very true. From a 2'x6' indoor enclosure when just a baby to a 3000 square foot outdoor pen (with some smaller pens along the way) he has used the entire space.
I used to live on a field that was 2 miles across, adult male sulcata would walk across and back with me every day in the summer time eating all the grass he could along the way. And stil would be upset when in went back in his 1 acre pen!