There is a very important training concept called periodization. Periodization was first widely used by the Soviet Union for its Olympic team. We use this same concept when training bat speed.
The idea behind periodization is that you must train at sustained levels for an extended period of time and then give the body a chance to recover. Here we are talking about a timeframe over several weeks of training.
One of the most important aspects of power training is to vary the overload and underload. The range of loading is important. Obviously, maintaining good form and technique is important as well.
Typically, using three different swing loads, we find 150 to 200 swings in a session for a high school age player leads to maximum results using these techniques. As an example, we have taken high school players from a starting speed of 70 mph to 82 mph in four weeks.
By swinging heavier loads, the player is forced to find more efficient joint muscle patterns of activation. These would be the most efficient movements. Combined with swinging a lighter bat to train the muscles to fire at their maximum speed, bat speed improvements can be dramatic. From a training perspective, you should train using exercises with the same or similar motion and exercises that are as fast or faster than swinging a bat.