It depends on the insulin that you are taking, and the time it takes for that insulin to "peak." If I'm not worried about being very high, I wait the full two hours to test.  My doctor has always told me that Humalog takes two hours to fully peak.  If I wait, I can be sure that my blood sugar is probably relatively stable, so I won't take too much insulin as a correction. If I really feel like I spiked, I will test earlier, about an hour or and hour and a half after.  Usually I can tell if I will need extra insulin.  And of course, if I feel low, I always test right away and have a snack. :)