Before considering when to test, I would ask "why am I testing"?
If you are wanting to record something in a book and track progress, you don't need to test very often - just regularly.
If you are wanting to learn the impact of something on your blood sugars to adjust your behaviour, then the generally advice is to test just before and two hours after,
This could be just before eating and 2 hours later to understand what impact the food you just ate had to decide whether to avoid it in the future. It could be testing before you do some exercise and when you have finished to see if there is any benefit in exercise on your blood sugars.
But, to me, there is little point in testing if you haven't considered why you are doing it.