I think it is going to be an individual thing. My blood sugar could often drop as much as 5mmol/ml with an hour at the gym. I found for me, the easiest thing to do was to take my pump off for the gym and my bm would still drp a little, but not by as much as 5 in an hour.
It will also depend on what exercise you do, if you are just doing resistance stuff, non-areobic exercise, this can actually cause your blood sugar to rise, if you are just doing aerobic stuff, that is hat brings the blood sugar right down, so should probably aim to do a good mix.
Trial and error I thnk, be sure to take glucose tablets or drink with you and don't forget your bm monitor.