It depends. For non-diabetics it's an hour or two. For diabetics with serious insulin resistance it can be many hours. It may also depend on what type of carbs and what else was eaten. Your best bet is to self test your blood glucose after eating and see what happens.
Yes, I think there can be. After all, if you don't get back down to "normal for you" before you start your next meal, there will be something of a build up. Clearly, if you have a longish overnight fast, you should get a bit lower than in the breakfast-lunch interval. I think Jenny Ruhl on the blood sugar 101 site, criticises fasting blood sugar tests because these are the last to show raised levels.
Sally
Just to clarify from what I said above, I suspect that the carbs (i.e. a well chewed slice of bread) will pass through your digestive system in a few hours, depending on a number of factors, but the resultant sugars will remain in your blood until you are able to process them. This is where the backlog is for diabetics, which can ultimately lead to blood sugar readings off the scale - unless of course you have just eaten a dozen sticky buns, in which case you might be able to achieve it in one go.
I'm partly basing these comments on seeing how my husband's blood sugars came down when he jumped in at the deep end with a strict low carb diet. They seemed to drop in stages, as if a backlog was being cleared.
Sally
@numan43 , I think I have responded a different thread of yours a while ago about the "Last Meal Effect", or as I tend to call it enzyme lag? Well, I think this is last meal effect/enzyme lag in reverse. Once someone's body gets used to eating and drinking in a particular way, their body starts producing the correct digestive enzymes and hormones etc., to digest what it's used to eating. It can sometimes take a few days for the body to catch up on the new way of eating, whether it be increasing or reducing carbs.
How many new people do we see who post along the lines of, "I've been eating low carb now for a week and my bloods have hardly improved"? Of course there are very many reasons for this, but if our bodies have been running higher, and our body was comfortable at that level, it tries to keep the bloods up in that comfort zone, for a while, then gives in.
Obviously the foregoing assumes functioning pancreas and working liver.