A glucose tolerance test can be anything from 2 hours or up to 5 hours in duration, you have to fast overnight then when you arrive they test your fasting sugar levels, then give you a very sweet drink to consume, after an hour they draw blood again to see how your system is reacting to the sugar challenge, then every hour after that, it's not a bad idea to get insulin levels checked at the same time.
Prior to having a GGT, a doctor sent me for just an overnight fasting glucose test which come back normal, the doc at the time said I did not have any sign of diabetes, how wrong was he, i have since learn't that many full blown diabetic 2 people can have normal fasting sugars, so it is a pretty useless test that a lot of doctors seem to rely on when testing for diabetes.

best of luck anyway
Brian