It kinda depends and it gets complicated but I will try and explain.

We all know that carbs get converted to blood sugar but so does free protein (that's protein that has not yet been converted and stored/used by your muscles). So that's the basic premise we can start from .

Now, some carbs get "rapidly converted" to blood sugar - things like simple sugars (sucrose, fruitcose etc) and starches (as found in potato, flour etc - or if you prefer, foods with a "low glycemic index" get converted more slowly than a higher index). Other more complex carbs (say fruit/vegetables which contain fiber and sugars) get converted more slowly (and if you subscribe to the program - higher "glycemic loads" keep the peak longer).

To standardize things that's why we test our post meal sugars (PPG) at 2 hours after you start eating. That gives a decent look at "what's happening" and keeps measurements at least somewhat consistent, but it doesn't really tell the whole story because your sugars most often have "peaked" before that two hour point if you ate very simple carbs and may not even yet be peaked if you ate very complex carbs very high in soluable/fermentable fibers.

Sound confusing? It is. So as a general rule of thumb if you start your exercise at about 1 hour "after you took your first bite" you will "likely" be burning sugar at the "start of your sugar peak MOST of the time".

At 3 - 4 hours post start of eating, unless you just "lay around" your blood sugars have mostly gone back to your pre-meal levels (or unless your wolfed a truckload of carbs). So you kinda have a window in the 1 to 2 hour range where you can best effect your sugar peak if that's what you are looking to control.

The little chart below shows where the average "peak" occurs and the point where we typically test (2 hours) and then how your sugars continue to taper over time.

Well it can vary from person to person. But as a retired nurse who once worked in cardiac intensive care unit, here is what they taught us :

The old wives tale of not swimming or exercising within the 1st hour of eating is actual reality. When you eat, your blood is rerouted to your stomach to help digest, which means your muscles and other organs (including the heart) have less blood circulating during that 1 hour, Then all goes back to normal again.

Which is why when people have a heart attack or chest pains, it often happens within 1 hour of eating, or during shoveling or any exercise.

So I guess I would say at least 1 hour after a meal because of these facts. Make sure to hydrate while exercising also.

When I was able to exercise, sometimes I did it on empty stomach in the morning as I take medications that I have to wait between 30 min to 1 hour before eating. But be aware some people drop their sugar while exercising, in the fall when I went to the gym I had severe hypos twice, so I made sure to bring with me apple juice with me while I exercised, instead of drinking just water, as when I go to the gym it is usually 1 to 1 1/2 hrs, but right now I have herniated and torn disc in my back so exercise has not happened much last few months.

Some people it is the contrary their sugars go up in the hour following exercise, mine dropped after 20 to 30 min, which is why I stated it varies with people, as we all have different metabolism. I was carrying my glucometer in the locker at the gym and tested before and right after, as it can be dangerous to drive should your sugar levels be dropping. I now use the Libre, even if it is not always accurate, you can see wetter the numbers are going up or down. Mind you here gyms are all closed as we have been in lock down since mid March

Hope this helps you