Humalog is very fast acting. It will start working in about 10 minutes. It will peak in about an hour and it will totally be out of your system in 4 hours. When people say it may "hit you later", it's because the insulin has lasted longer than your carbs have. As long as you are eating enough carbs to cover the amount of insulin you have taken, you should be ok if you stay on track with your meal timing. That means, eat a meal containing the right amount of carbs before you take your insulin (10g per 1iu) and follow that up with another meal every 2 hours for the life of the insulin. Even if you do everything textbook, you still may find days that you begin feeling hypo. Keep some simple sugars on hand, glucose tabs, honey, dextrose powder and use that to pull yourself out of a crash. Then go straght to your next meal. Homalog is easier to use because its life is shorter, but it can still cause issues and still be just as dangerous as a longer acting insulin. Be safe and do not take it if you plan on going to bed or nap within the active life of the insulin.