Rest between Sets

In general, rest as long as it takes for you to feel recovered from your previous set. That normally ranges from 45-90 seconds. Larger muscle groups take a bit longer to recover; smaller muscle groups clear low pH levels are are ready to go more quickly. Don't fall into the all too common mistake of talking with your buddies for 3-4 minutes between sets, during which time your muscle can become cold. This is counterproductive and lengthens the time you spend in the gym. 

 If you want to emphasize strength, take a little longer rest between sets. On the other hand, less rest means you won't be able to lift as heavy, but you'll be stressing your endurance. Of note: How much you can lift on a given set and the number of reps you do are directly related to the length of your rest period.