muscsportsguy saidMy two cents:
1. Do abs more often - you can work your abs without the risk of overtraining. Just mix up the exercises.
2. Do more leg work. Your bigger muscles - in particular your glutes - can be worked more often than others. Just don't do a muscle group two days in a row. Recovery is also important.
3. You can work legs into almost any workout. For example, while you're "resting" in between bench press sets, do box jumps. This is a great "power move," which will keep your heart rate up and give you extra work on one of the major muscle groups.
4. Throw in some compound movements. For example, on days when you're doing shoulders, do a squat up to a shoulder press with a dumbbell. Again, working major muscle groups more often, and the more minor muscle groups a little less often. This will give you extra work on the big ones without forcing you into the gym more often than you want.
Anyway, take that for what it's worth.
1. If you are lying on the floor doing body weight abs, yes. If you do weighted crunches, they are no different than any other muscle you exercise using resistance. Floor crunches are an endurance exercise for most people on this website.
2. It depends on how intensely you work them. If you can do it again in 2 days, you don't lift heavy. I can sit down in chairs every day multiple times and that's body weight. I could do 135 everyday if I wanted to. I can't do 315 everyday. Same thing as the abs except you actually use these muscles everyday to sit down and stand up, or have sex.
3. Most people's goal is not to get a cardio workout while resistance training. If it is, keep doing your sets in between sets. If you care about gaining muscle, you need rest and central nervous system drive so you can lift a max effort during your lifts. Doing legs in between taxes the nervous system.
4. It sounds like you do circuit training. That's fine if your goal is resistance training while getting some cardio in, but you aren't maximizing either.
To answer the OP. If the muscle hasn't recovered (Ie not doing able to lift the same weight or more next time), you're training too much for making gains. Muscle gain happens when you lift more weight consistently. It's all about trying to lift more weight over time, for both hypertrophy and max strength training programs. There's no magic number of days--it depends on how good you are at recovery nutrition, sleep, and preventing overtraining. I personally don't find the need to train to exhaustion in most cases. That's just too much breakdown and drains energy and causes muscle breakdown beyond what is needed to make consistent gains.