How fwr can I go ?

Depends on your riding style, and the jetting. How long can you ride on a tank? Ride around like you normally would on the shorter trails (preferably known distance trails), and see (1)how much gas you actually consume riding a given distance, and (2)how far you get before needing gas. I think I can go an hour and a half on MX tracks on my KX250 with a similarly sized tank, but I have never needed to check (I'm usually exhausted before the tank runs dry).

Unlike cars there's not really a MPG rating. Mostly due to varied conditions and styles of riding. Slots of tire spin doesn't get you too fare but burns lots of fuel.

The first thing to do is to make sure how big the tank is. Any estimate will be off if the wrong tank size is assumed.

Next, start with a full tank and ride for a while. Don't try to stretch the distance, just ride normal and get back to the truck with plenty of gas.

Now refill, carefully measuring the gas it takes to refill it.

Now you will have an idea of how much gas you used for however long you rode. Use that to estimate your full tank range.

Then only ride 3/4 of it.

Rod