IMHO...
1. 4-7 tanks
2. 3 mins to 15+...depends how you drive
3. Assuming you want a nitro st, a fuel tank capacity is 75cc. One US gallon is 128 fl. oz or 3,785.4118...cc's. So, there are approx. 50.4721573 tanks of fuel in a gallon. For a 125cc T-Maxx tank, its about 30.2832944 tanks.
4. I have about 15 tanks through my RC10GT, and still have to burn out the stock plug...did break in with the same plug as well. But, I've seen plugs last two seasons.
5. Pistons/Sleeves/Connecting rods are fairly inexpensive for stock engines...with a high quality engine (OS etc), its essentially cheaper to get a new engine. Other than that, you'll need after run oil, and air filter oil, and other stuff, which lasts quite a while...I've been using the same bottle of after run oil in all my nitro RCs for over a year.

6. Its handy to have a temp gun, but the water drop method is suffecient enough...but a temp gun would be A LOT of help...
7. You'll need a new piston and sleeve about every 3-8 gallons.