Rest. It’s a term many serious athletes come to hate. Training gets addicting for many athletes, but too much training will take a toll on even the most healthiest of bodies. Everyone needs rest, no matter the level of the athlete. Rest is crucial for recovery, as exercise/training actually damages the body (in both good and bad ways, depending on how you train). Without enough rest, your body will keep trying to heal without enough resources, kind of like pouring water into a cup with a hole on the bottom of the cup. How much rest is enough, too much, or too little? That will depend on each person.
Age. A younger body will need much less rest than an older one. This one’s pretty common sense to most people, but it is something that still needs to be mentioned, even though it’s the elephant in the room that nobody wants to come to terms with. Somewhere in your 20’s, you might start to feel your recovery time change. And then again in your 30’s, etc. Listen to your body, and give it ample time to recover. Don't compare yourself to someone older or younger than you when it comes to training or rest. Or anything, really. Age really is but a number, but generalizing, it can actually say quite a bit about a person.
Gender. Gender is not an excuse for anything, but lower amounts of testosterone will actually affect recovery time. If your workouts involve building muscle, you will be increasing your testosterone level automatically (women, it has a natural cap, so you don’t have to worry about getting to the bodybuilder look).
Injury. If you are recovering from an injury, a simple general rule of thumb is to wait an extra 1/4 of the time it took to recover to 100% before you start training again. Example: If it feels better in less than a week, wait an extra day after it feels 100%. If it takes 4 months, wait an extra month. Many chronic injuries (injuries exceeding 6 months in duration) are actually minor injuries that were extended because of improper recovery time.
Diet. If you are eating a diet of foods your body has a hard time processing (junk, allergic/intolerance foods, artificial anythings), you will have a slower recovery time. If you are not drinking enough water, you will have a slower recovery time. If you have circulation problems (like high cholesterol, high blood pressure, varicose veins, etc), your body will have to work much harder to distribute the nutrients it needs to recover. If you are taking supplements, those can either hinder or help your recovery. Typically, supplements that help with your actual workout will slow your recovery.
General condition. If you haven’t worked out in a while, it’ll take you much longer to recover from a workout than when you’re already in shape. Either way, don’t let muscle soreness get in the way of your training, but if you start to feel any signs of overtraining, you should rest.
How your body responds to certain stimulus. Some bodies are designed for the slow twitch stuff, like long distance running or 90 minute jumprope sessions. Others are more designed explosive plyometrics like striking or burpees. Have you ever ran for 60 minutes and been unable to walk for the next day, while someone else was completely fine after doing the same? Our muscles are programmed to be able to handle more of one type of training than another, and it will differ among athletes. Yes, this is something that can be trained, but there is already a predisposition to what your muscles can handle.
Type of workout. If you are used to that type of workout, your body will take much less time to recover. If it is something that requires you to use different muscles than you normally do, or just do something in a different way (aka, “mixing it up”), it may take you longer to recover.
Frequency of workout. With a weekly regimin of 2-4 evenly spaced workouts, extra rest is probably not needed, as there should be enough time between workouts for the body to recover. However, for more serious athletes training 2-3 times per day, actual rest days should be planned into the week.
How long you have been training consistently in your life. If you are new to the world of physical activity and competitive sports training, yet find yourself addicted to the mats, you’ll have a little more difficulty recovering than someone who’s already been in the game for longer, even if they’ve never trained that sport before.
Types of rest:
Active rest. This can range anything from a light version of what your normal training is (maybe a 1-4 out of 10 on the physical exertion scale), to stretching/yoga, an easy hike, or just playing outdoors with the kids. You should really be giving the muscles you normally work a break, but while still doing something active. This will prevent things from stiffening up, but still be providing the body with some sort of physical activity to aid in circulation, hence speeding up recovery (blood circulation is how the body delivers all the good stuff to the cells to repair them).
Passive rest. This is where you actually do nothing. Like sleep. Watching TV doesn’t count, because it’s actually wasting your time (your body burns more calories from sleeping than it does while watching TV, since your body is actually working hard at repairing itself while you sleep, but not when you watch TV). Naps are great, especially if you are doing two-a-days (or more). Some people need a mere 6 hours a night. Others need 12. Some need naps, some don't. Whatever it is, be consistent, and your body will know what to expect and do a better job of recovering.
So how do you determine the amount of rest suitable for you? Trial and error. Since each person’s situation is different, and each body will react differently (even to the same stimuli), try to go to bed at the same time every night, and wake up at the same time. Do this for a month straight, and see what happens. You might need to add a nap during the day, or cut out an hour of sleep in the morning. Once you get into the habit of proper rest, your body will recover more effectively, and you'll feel much more refreshed every day.