"Average" is of no use at all. You want to be able to get home. Worst case (i.e. 5 ft head sea) is a better one to consider. I run a 23' GW w/ T150 Merc OB's and typically use ~ 90 to 100 gals. for a days fishing (mostly trolling on one engine) about 50 to 60 naut. mi. out. My boat carries about 180 gals of fuel, so I'm OK with that. You need to base your judgement on your boat's fuel consumption, and a good way to start is not to go way out until you're comfortable with your fuel situation. Of course, fuel is only one consideration when running offshore, but that's onother issue entirely.
WildOne has spoken some good words here. Also, what you should do is to top off your fuel tank(s) and then run at cruise speed for 1/2 hour and then turn around immediately and come back for 1/2 at cruise. Then go back and top off your tank.
This will then give you a rough idea of gallons per hour. If you do this on a calm sea expect to use more in a rough sea. For instance, if you're getting a burn of 14 gallons per hour on a calm sea then expect about 16.5 to 17 gph on a rough sea.
Good rule of thumb for running offshore is figure what 90% of your total fuel capacity (ie; if a 100 gallon tank then you have 90 gallons at 90%). Once you've got this figured out then allow 1/3 out 1/3 back and about 1/3 for trolling.
If you burn 14 gallons per hour at cruise and you are moving at 28 miles per hour then you're getting 2 miles per gallon. So at 1/3 of the tank that's 30/14 = 2.14 hours at 28 mph or you can go about 56 miles offshore (but I'd leave it at 50). If you burn about 6 - 8 gallons per hour at troll, then you have about 4 to 5 hours of trolling.
Like WildOne said this is only the start of going offshore. I wouldn't try it until I really knew my boat, it's handling characteristics, my ablility to handle unexpected bad weather (ie; thunderstorms with high wind that come up unexpectedly) and so on............ this is another topic as well as a couple of other considerations. Remember, there's no pulling over to the side and going to get help like in your car. You cannot get home real fast when moving only 28 mph and the storm is coming at you at anywhere from 35 to 50 mph. Being mechanically inclined as well as knowing first aid etc, is a big plus out there.
Try what I said about your gas consumption and see how you feel with it.
I have a 100 gallon tank, burn 14 gallons an hour (hmmm sound familiar? Hehe) and I cut myself off at 50 miles offshore. If I should dedcide to go to 60 then I cut my time fishing (trolling) back accordingly. [excited]