how about like this?

1. 3 mice eating #1, 1 mouse eating #2. (20min)
2. 4 mice eating #2, 1 mouse eating #3. (10min)
3. 4 mice eating #3, 1 mouse eating #4. (12.5min)
4. 5 mice eating #4. (9.5min)

total time = 52min