I have had I think six glucose tolerance tests!
The first was two hours as described by
@bulkbiker, the remaining five were extended oral glucose tolerance tests.
They sometimes do half hour finger prick and sometimes every fifteen minutes.
There is a crucial difference between the two.
Two hours test, you are expected usually to have carbs previously to the test.
With an extended test, it always fasting.
Depending on what they are looking for, depends on how the test is done.
There is more than insulin resistance or insulin production (when and how much?) That is measured because during both tests, blood is taken.
And of course because of your previous full panel blood results, depends again, on what they are looking for.