If you've been away from running for less than a week:It's possible to take up to a week off without losing any ground. In fact, a few days of rest may even improve your performance, especially if you've been feeling exhausted and sore. But after a week of not training, you'll quickly start to lose your fitness -- a lot faster than it took you to build it up.
If you've been suffering from a cold or other short-term illness, make sure you're healthy enough to get back to running. The general rule of thumb for running with a cold is that if your symptoms are from the neck up (sore throat, runny nose, etc.), then it's OK to run with a cold. If your symptoms are below the neck (chest congestion, diarrhea, etc.), it's best to wait until you're feeling completely healthy.
If you haven't run for less than seven days, do NOT try to "make up" the miles that you missed. If you try to squeeze all your missed miles into a short period of time, you could be at risk for a running injury due to overtraining. Just pick up your training schedule where you left off. You may feel a bit sluggish during your first run back, but it should only take one or two runs before you're feeling like your old self.
If you've taken one to two weeks off from running:
If you've been out of your running shoes for only a week or two, start at about half the distance you were running before the injury. If you were recovering from an injury, go easy when you first return to running because if you run too hard, you risk re-injuring yourself. You should be able to build back to your former level in two to four weeks.
If you stopped running for more than two weeks:
With a layoff of more than two weeks, you need to be conservative when you return to running. Chances are you haven't run for more than two weeks because you've been injured, so make sure that you're definitely ready to come back. If you've been under the care of a medical professional, make sure you get cleared to return to running. If you're not ready to come back, you could possibly cross-train in the meantime, if it doesn't affect your injury.
Once you're ready to run again, don't assume you have to run your entire distance. Start with a run/walk approach. As you build your endurance, you'll be able to extend your running segments and reduce your walking time.
In the beginning, take a day off after every running day. For your weekly mileage, you'll need to gradually work up to your previous level. And don't keep making jumps in your mileage. It's good to get comfortable with a specific weekly mileage by staying there for a couple weeks, and then bump up your distance.