About how much riding time can I get on one tank of gas on a 2007 yamaha 450F ?

Either ask someone who is running the same class or do a test run as that is the only sure way to know just when you need to refuel. Many who do the longer races or enduros tend to put a bigger tank on the bike. I know the modern 250fs get about 35 minutes or perhaps more to a tank depending upon the type of riding. Doing 30 minutes plus a lap motos and usually a sight lap has the tank with very little left in it but each bike can differ a bit depending upon how well maintained, tuned and how you ride it also track conditions. Get yourself one of the enduro fast fill cans (which are often expected to be used along with environmental mats for enduro). Makes refueling stops a bit quicker. Best to read your supplementary regs as to what is allowed and what is not as many hare scramble races also won't allow the use of tear offs due to environmental reasons and you need roll off for the goggles instead.

Taycee told you correct.

A lot depends on your riding style. If you are flat out most of the time you will only get about 40 minutes out of the tank (the tanks fuel range is set up for modern pro motocross - 30 minutes plus 2 laps or right around 35 minutes).