NM - when are you thinking of going? Not all the glaciers are open all summer. The biggest one is at Les Deux Alpes in France, but it doesn't reopen until about the end of June. Tignes, also in France, is open longer but it's a smaller glacier area, and a less attractive place in general. In Switzerland best bet is Saas Fee, or in Austria try Hintertux.
Consider where you're staying relative to the lifts - when you're aiming for a 7.30 or 8 am start the last thing you want is a journey to get to the lifts first. Usually you can ski until between 12 and 2 depending upon the time of year, but I've known weeks in June when it's snowed continuously and all day skiing is possible, as long as the lifts are kept running, which they won't be everywhere. Les Deux Alpes is good for afternoon skiing when the conditions are right as they run the funicular all afternoon for tourists to visit the ice caves.