its a loaded question as HOW far can you go, depends on a too many variable's to give a accurate answer here
my suggestion would be, fill your tank FULL full, rid it for say 25 miles and take it to have it filled back up to the top,full full, and do the math, and that will get you a ball park on how far your going on a gallon of fuel
NEW atv;s do NOT have a reserve tank, most all only have a idiot light that shows on the display when they get to a certain point(guessing at about a half gallon or so)
I would think MOST modern ATV's have a MIN of a 40 mile range, pending HOW you drive it, some may go a lot father, but I think ALL will make it about 40 miles on a tank
its pretty easy these days to carry more fuel with you, if you travel far often?