It really depends on what you start with. A 5/8 cut will go pretty quick but a 7/16 will last longer. This does not apply to kids however. I swear my son must be walking on rocks by the way his skates get nicked up...
You may consider spending some time trying different cuts: 5/8, 1/2, 7/16. You will notice more "bite" with 7/16 but it can take some time getting used to how deep it is (makes it harder to stop if you are a beginner).
If you can't figure out when to get it done, just bring them in every 10-15 skates and try out the range of cuts. Eventually you will settle on what works for you.
For me, I now use 5/8 because it gives me the edge I want without the break-in period. Now I take them in about every 10 skates. If they get beat up, I'll take them in sooner.
Here are some old threads on the topic as well:
How many games between your sharpenings?
Preferred skate blade hollow?
Skate Sharpening Radius Advice