Digestion starts as soon as food hits the stomach. You can test 1 hour after your first bite...or, you can test when the cows come home. Take your pick! It all depends on what you're looking for. If you're looking for your highest spike...take a test every 10 minutes (taking into consideration your meter's 20%± accuracy) and that will give you a nice graphical curve to whatever you slipped down your pie hole during that meal.
As you've alluded to...consistent test times, which I feel also, are more important to your overall judgment in how you should deal with these numbers.