Keep an eye on the led, when it starts flashing red you will have a minute or two before it auto lands. With a new fully charged battery and using the stock 8 inch props you can expect about 11 minutes before the first red led warning and you might get about 13 minutes before it auto lands, assuming there is no camera or other attachments. Using 9 inch props (9443 or 9450) you will get an extra two minutes, about 15 minutes total.
The best time I ever got from the Phantom 2200 mah battery was exactly 16 minutes till auto land, 9443 props, no camera.
It's a good idea to label your batteries #1, #2, ect and keep a log of how long they last. Over time it will decrease. Use a stop watch or timer while flying and bring it back close to home a few minutes before it's due to auto land.