When my sister was running a KX100 in AMA D-7 harescrambles (2-2.5 hours long) she had to make a pit stop half way through the race. I'd say anything over an hour is pushing it. Of course your fuel range can vary greatly depending on conditions, bike set up and riding style. You just might be able to make it without stoping, but it would be close. Do you do any practice riding at all? If so you should be able to get a good idea of your fuel useage from that. If you decide to go without a pit stop consider carrying a small amount of fuel with you in a tool pouch or something. Though not exactly kosher, I used to carry a gatoraid bottle of pre-mixed fuel with me in harescrambles, just incase I didn't make it to my pit.
good luck and let us know how it goes:)