If wishing to check your bolus is correct.. Ideally 2 hours. (If your levels aren't good betwen 2-3 hours after a bolus, then generally it is your bolus or carb counting that would need attention.
Generally 4 hours is when your insulin from a bolys should be pretty much out of your system. However this varies with people.
My consultant told me to get basals right. Do a bolus test and ensure I checked 2, 4 and 5 hrly to find out how long insulin bolus lasted in me. This consultant advised to leave 5 hours between meals to stop bolys stacking and to find out how bolys's and corrections worked with my body.