Certain exercises are far more demanding on the body than others, and thus require more rest between sets. For example, a set of 15 reps of BB Squats will not only tax the thighs, glutes, and lower back, but will also have you breathing like a freight train. You may find that you need a good 3-4 minutes before you are ready to get to the next set. On the other hand, performing 15 DB Side Laterals may only require 45-60 seconds of recovery time. Another point to consider is what your primarily goal is, and how weight training is best utilized to reach it. Someone who is in the gym to build massive size and power will want to rest for longer periods in between sets so as to be able to lift maximum weight for maximum reps. When one is looking to burn body fat and perhaps increase endurance, it is best to keep heart rate elevated and move rather quickly from set to set, even if weights utilized are lighter. As for time in between workouts, you may find that as a beginner, that a full day of rest is needed in between weight training sessions. However, as you progress and become more accustomed to the rigors of training, it is possible to lift for several straight days without a break - just as long as you never work the same muscles two days in a row. Cardio, however, can be performed daily.