15 to 20 minutes per pound at 325 F. You really need an instant meat thermometer. 185 F thigh, 170 F breast.

If you are in the UK, use this guide
http://www.britishturkey.co.uk/cooking/times.shtml

Epicurious has a great chart for turkey doneness, taking into account turkey size, oven temp & time: http://bit.ly/dMQo7M But it's best to take the internal temperature too -- you'll want to hit at least 150 in the fleshy part of the thigh (the temp will continue to climb as it rests).