When should I notice the effects of a diet on my hypoglycemia?

Very quickly. I would expect an improvement almost immediately.
Right away. If the diet is working, you should no longer experience hypoglycemia. A very important question is why are you having hypoglycemia. Is it documented or do you just feel hypoglycemic. Please see a physician to get this evaluated, otherwise, you might be just treating a symptom and masking the cause.