I read another post and someone suggested the following plan for figuring out how fast your insulin begins to work. I tried it with my daughter, and I have consistently found it to take 30 minutes for her bolus to take effect.
Bolus for food intake.
Wait five minutes and check again. Continue to check BG every 5 minutes until BG comes down 5-10 mg/dl. When it comes down, you'll know how long it takes for the insulin to begin working.
I would always be very careful when you do this so that you are able to eat as soon as it comes down to avoid a low, but I found it quite helpful.