My DD did 25 hours/week last summer (she started last summer at 7 years old, turned 8 mid-summer). There was a lunch break every day, plus an hour of ballet/dance once a week. No extra time needed for TOPS/conditioning, that was all embedded into practice. Actual gym time was probably closer to 21 hours per week when you take out breaks and dance. The hours were totally fine for my DD - not too much. Certainly less tiring than during the school year (when she has a full day of school followed by a 3.5 hour practice most days).
In fact, the summer before that (when DD was in Xcel at age 6 turning 7) was actually harder. That summer DD went to a girls sports camp from 9-3 everyday and then went straight to Xcel practice from 4-7 PM / 3 days per week. Less gym time but longer days. Compared to that, 5 hours/day with breaks was a breeze!
ETA: Of course, it depends on the kid! One of mine would not be happy with that level of activity at any age...