1. Sleep for more than 7 hours.

2. Sleep more than 7 hours.
(Are both OK? Which one is commonly used?)

3. Don't exercise too hard.
4. Don't exercise too much.
5. Don't exercise too often.
6. Don't exercise too long.
(Can we use all the expressions?)

All are okay, but 2 is better than 1.

3. would be like using weights that are too heavy.

4 would probably relate to frequency or duration of exercise.

5 would relate to frequency.

6. would relate to duration.