I would guess it would depend on how the course is set up , if there are a lot of slow technical sections , or a lot of high speed sections , or a lot of deep sand washes to rob mileage , that will determine how the fuel is used up , i would guess you should expect about 20-30 miles on a tank at any kind of race pace i am guessing , but that again depends on the speed average of each lap , so i would say at the very least you will have 3 pit stops , it would also depend on how many crew members you have , if you have 3 or 4 guys , then i would set up 3 fueling areas along the course that way you can just pull in top off and go losing minimal time , unless they only allow 1 designated pit area , if that is the case , then just plan on doing fuel stops every 2 laps , but i would not push it past 2 laps , or you might just end up pushing it to the next pit , you may lose a slight bit of time , say 30 seconds per lap depending on the gas jug you use **I use the Tuff Jug , it has a sort of quick-fill setup , you just hold the can inverted and push it into the opening and when the gas hits the lip , it stops fuel flow , and you pull it out without spilling a drop and i can do 1.5-2 gal , in about 5-8 seconds** for those stops , but you would be safe to not run out that way
http://www.tuffjug.com/ , they come in 2.5 and 5 gal , i put my Av Gas and Premium in the 5 gallon ones , then mix them in the 2.5 gal cans and those are what i take to the track as the 2.5 are so much easier to lift up to the tank and hold it than a 5 gal
Do they allow you to pre-run the course to get a feel for the layout ??? , if so you can fill up , run thru the course at a mediocre speed and re-fill and see how much fuel you use in 1 lap , that should give you a good guideline , since under race conditions you would subtract about 1/4 of that distance to get a good idea on fuel mileage , i don't race off-road , so i am just giving estimates , but i know when i do motos on a GP type course (rough whooped out 3rd gear stuff) , i get about 5 laps per moto , at about 3.2 minutes per lap , for about 15-17 minutes , i believe its about a 2.3 mile lap, and do 4 motos on Saturday , and the same on Sunday (I will be 59 next week), and use about 1-1/4 gallons of gas total for each day , so i can do all 4 motos on 1 tank of gas and end up with about 1/4 - 1/2 a gallon leftover on my 2.0 gal stock tank , but that is with a Carbureted CRF450R not one with EFI , so yours would better that by at least 1 quart or so ...................... hope that helped somewhat .....