The general guidelines are to test 2 hours after first bite, and this is the only "target" guideline we have. It seems to have arisen from the 2 hour rule for the OGT tests, which is 2 hours after starting the drink. Of course, this 2 hour marker will never be the peak of the rise, and will never show us how long we have had elevated post meal levels, which is why it is sensible to test new foods or certain other combinations of foods more often than just the 2 hours. The 2 hours is simply a guide.
I had an occasion yesterday when I had a very fatty meal with not a lot of carbs.
At 2 hours I had risen just 0.2mmol/l.
At 2.5 hours I had risen 0.7mmol/l
and by 3 hours the rise was 1.0mmol/l.
Then back to normal