How long can you fly with a full tank....that depends on how big the tank is, how rich the mixture is set and how much throttle you're using. Figure on 8-10 minutes...(at a guess!) with a little reserve left to land with. I don't know of any easy way to tell if the tank is almost empty...most people use a timer of some sort. Set it for a number of minutes and when it runs out, it's time to land!

How long the battery lasts will again depend on how much you move the servos, the battery's "health", temperature etc....if you're moving the sticks back and forth like crazy, you'll use more power than if you let the airplane fly itself and just make gentle corrections. Best bet here is to get an expanded scale voltmeter that will let you check your battery under load. Check your battery after a couple of lights and see how much it's dropped. Most of the meters out there will display "OK, Recharge" etc as a visual warning that its time to recharge. There are also a number of devices that plug into a receiver channel and "beep" or light up to indicate battery condition.

As for the ailerons, with the aileron trim centred, are the ailerons both even with the wings? If not, adjust the control horn and/or the linkages to get them centred. Once you've done that, try your ailerons again. This is one of those problems that is easy to demonstrate than to describe. If this doesn't work, you might want to post some pictures or get an experienced pilot to look at your setup.

Lastly, I'd really suggest getting an experienced pilot or instructor to look everything over and to (at least) stand beside you for your first flights. It's really quite amazing how quickly you can get into trouble!

Good Luck!