One of the most common questions about weight training aka resistance training is how long should you rest between each exercise set? If you take a look at the relevant research concerning this topic there are some good answers.
For a long time, researchers believed that shorter rest periods were better for increasing muscle size which was thought to occur because of increased levels of metabolic stress by not allowing complete recovery. However, research done in 2015 shows that using longer rest periods actually results in better results for both increased muscle size and increased muscle strength. It is thought that this occurs because longer rest periods allow a higher training volume (more repetitions of each exercise).
Another aspect to consider is whether most of your workout uses compound exercises (compound exercises use multi-joint exercises using more muscle mass such as a chest press vs a pec dec or pull-up vs an Lat Pullover). In a 2012 study, researchers found that with shorter rest periods workout volume decreased for both single-joint and multi-joint compound exercises. However, the decrease in volume in workouts with short rest periods tended to be significantly greater with multi-joint compound exercises vs single-joint exercise (29 percent reduction in volume with multi-joint as compared to 15 percent with single-joint exercise). So for strength routines focused on compound, multi-joint movements increased rest time is probably a good idea.
Another study which looked at the effects of rest interval length on training volume discovered another important item: reduction in training volume occurs mainly between 1- and 2-minute rest periods. The percentage difference between 2- and 3-minute rest periods is much smaller, except when doing 5 or more sets of an exercise. So, for people doing 3-4 sets per exercise, 2 minutes is a good rest interval, but if doing 5 or more sets you should probably extend the rest interval to 3 minutes.
Another important piece of information is that recreational lifters who self-determine rest periods tend to rest just under 2 minutes between sets meaning that for most lifters doing 3 to 4 sets of several exercises whose goal is increased muscle size and strength you probably do not need to time your rest intervals and can go just go by feel.
Timing rest intervals becomes more important for advanced resistance training such as Power Lifting and Bodybuilding, but for most of us we can just go by how we feel!