As Sally says when to test depends on what information you want?
If you want to know how high your BG went after eating you first have to know when YOU get the highest BG. Each of us is different and we dont react the same to food.
I took my normal meal and 1 time I tested every 15 minutes after eating. I found I spiked at 45 minutes after I eat.
So for me to test how High I went I test 45 minutes after I eat.
If I want to know how well I over all handled the meal I test 4 hours after I eat.
In order to see the maximum blood sugar, the time that is recommended to check is about 1.5 - 2 hours after starting to eat. At this time you don't want to see a blood sugar higher than 140, though < 120 and even in the 90 - 100 range is totally normal.
If you want also to see if your blood sugar goes down to normal after eating, then you check just prior to your next meal. At that time blood sugar has ideally returned to fasting levels, typically in the low to mid 80s.