Moke, a normal meal may take four hours to digest totally but starts to digest much sooner so you need insulin in your system when the food starts to digest. The insulin stays in your system (depending on what type) for several hours. You may need to test your blood sugar two hours after eating occasionally and if it is high or low, you can talk to your doctor or diabetes educator about adjusting your insulin.
With certain foods such as high fat hot dogs and pizza digestion slows down and you may find that you have a low blood sugar an hour after you eat and then will be high later. This is because fat slows down the digestion process. A meal like this may be a little harder to figure out unless you are on an insulin pump.
Find out from your educators what that two hour post prandial blood sugar should be and you will know if you are taking the right amount of insulin when you eat.
It often takes hours for all our food to be digested, but carbohydrates are digested more quickly. Simple sugars are digested even more quickly & some are initially digested in our mouth. Fats & proteins take longer.
Depending on what we eat, we could get a blood sugar rise in just a few minutes or -- if it's complex carbohydrates mixed with proteins and fats, the rise will be more gradual and over a long period of time, as you mention.
I'm not a physician or nutritionist, but this is how I understand digestion to work. I try to take my insulin (dose & timing) based on what's in the food I'm eating.