My check engine light comes on the day after I get gas. I have had the light diagnosed and it comes up as fuel line. the fuel line has been visibly inspected and is in good shape. the mechanic thinks it is air in the gas tank. If i wait it out the light goes off in 4 days. What should I do?
What you need to do is post the exact code that came from the diagnostic. This will be in the form of P0123
Your mechanic may have written it down somewhere and/or it could be on your invoice. If not many auto parts chain stores will ready your codes for free.
There is no code that will tell you anything about a fuel line. It is likely that it was an evaporation system code, however. Thoroughly clean your gas cap and the rim of the filler neck. Maybe use some rubbing alcohol on a rag. Coat the gasket part with a small amount of vasoline or something and make sure it clicks several time when you put it back on. If this clears it up buy a new gas cap. Or just buy a new gas cap anyway - they’re cheap.
I agree with cigroller. Get us the number. I have to wonder if you understood what your mechanic said. Air is always in the tank. My guess is you have the one thing we all fear. A bad gas cap. You can purchase one at the autoparts store for a few $$ and with some advice you might even be able to install it yourself. 
Are you putting the cap back on tight (letting it click at least once?)
Are you stopping on the first click or do you fill it to the top? If the latter, thats your problem.
" What should I do?"
Nothing…Drive on…
I agree that we need the code.
As gas is pumped out, the gas tank breaths air in to replace it through a system called the “evaporative emissions system”. If that cannot breath air in, the system will detect too high a vacuum building in the tank and trip a light. Perhaps this is what the mechanic was telling you.
The EVAP system is basically a line from the tank to an activated charcoal bed in a canister through which the tank breaths. If that charcoal bed becomes saturated it can restrict the ability of the tank to breath in. That can happen in some vehicles by topping off the gas tank. Do you do that?
I have a 2003 XL7. I put a locking gas cap on and it sometimes causes the
"check engine" light to come on. If the gas cap does not seat properly, the light will come on. Clean the cap gasket or get a new cap. That should fix your problem.