It depends what my BG is and how much insulin I have taken,
I am more concerned by my Bg when I start and the calculated impact of any insulin I have on board,
I check my BG is between 4.5 and 8.5 before I start.
If I have just eaten and I am confident with my carb counting, I could start exercising straight away.
If I ate an hour ago, my BG was 4.7 and I had a unit of insulin on board, I would munch a GluocTab to raise my BG and carry on exercising.
If I am planning my exercise and food, I would target exercise 2 hours after eating but would be happy to take a correction dose closer to the exercise time.