A lot of people carry around the erroneous belief that in order to grow you must never take any time off from training. This couldn't be further from the truth. It is important to rest for bigger gains.
Weekly Rest Days
Each week you have a training schedule (or should have a training schedule). One of the scheduled days should be for resting. That means no cardio and no weight training. Hiking or biking would be OK, but no weights and no extreme cardio on your off days. This is very important for your body to grow.
A Week Off Every 12 Weeks
Most training cycles last 12 weeks in length. Once you complete a 12 week training cycle it is important to take an entire week off from training. It is OK to do some light cardio, but absolutely no weight training during this week off. Many people are afraid to take time off like this because they think they will lose their gains. This is patently false. I have personally found that my strength actually increases after a week off. The body needs to recover and after a period of continuous weight training your recovery slows way down. The week off gives your body that much needed recovery time that will enable you to make bigger gains.
Listen to Your Body
While 1 day off a week and 1 week off every 12 weeks is the basic formula, it is more important to actually listen to your body. Sometimes your weekly recovery just isn't happening. Perhaps you need some extra days off during the week? I've personally experienced this issue when doing High Intensity Training. I was lifting full body 3 days a week. The second to last exercise of each session was deadlifts. After 3 weeks of this I began to dread the deadlifts. After speaking to my mentor, I was told to cut back to 2 days a week instead of 3 days a week. I was shocked, but listened to the advice. All of my lifts increased with the added rest.
Additionally, some people require a week off after every 6 or 8 weeks of training. 12 weeks is merely a guideline, not a rule. If your body says it needs a week off, take that week off.
Resting Brings Bigger Gains
We don't really grow in the gym, we grow outside the gym. Getting enough sleep, taking enough rest days during the week and taking an entire week off every so often are the keys to getting the rest you need to bring bigger gains. It may not make sense, but it is a proven principle. How often do you take a week off?