To certain types of players, rest is really important. The way I play, I've learned that there are three levels of rest which I need to incorporate at all times AND I've been experimenting with a fourth level.

                    Level 1

The first level is the beat level. If I practice a phrase which is 50 beats long, then I wait 50 beats before I repeat it or go on. To maximize my use of time, I sing and finger instead of counting. So, for example, after playing this 50 beat phrase, I sing (while fingering) the exact same phrase. This insures the right amount of rest while at the same time keeps my mind working on the music. It's good for ear training and right hand technique.
 

                    Level 2

The hour/minute level. If my practice session is twenty minutes long, I wait twenty minutes before starting the next session. I've heard that Maurice Andre does something similar. I've heard that his first practice session of the day is one hour long. The second is an hour later and a half hour long. The third is a half hour later and fifteen minutes long. The rest of the day, he does fifteen minute practice sessions at fifteen minutes apart. this may not be true, but it works for me.
 

                    Level 3

The Day level.  I alternate days of difficulty. Actually, I only push myself hard two times per week. The other days I consider lesser, resting days, even though I'm still practicing. Plus, I take at least one day off, completely, per week.

On the days that I call "Big Days", I'll practice for as many as eight hours, if I have the time. On those days, I play everything up to super "C". Everything includes, long tones, lip slurs, articulation studies, multiple tonguing studies, scales, jazz patterns, etc.  On the big days I'll practice things like the Brandenburg #2, any other picc. stuff, and anything else that requires lots of physical endurance.

On the "Small Days", I practice for as few as thirty minutes or as many as two hours. I don't play anything high or physically demanding. Compared to the big days, these seem like days off.
 

                    Level 4

I've also noticed that a good week is always followed by not so good of a week. I won't call them bad weeks, but I've been experimenting with alternating weeks of difficulty. It's something that has sort of been happening on its own. I'm just trying to learn how to use it to my advantage.

I don't say that this works for everyone. different players require different amounts of rest, depending on the way they play and what they want to sound like.