As a former baseball player I remember standing in the on deck circle, staring down the pitcher trying to time my swing according to what he was throwing. Depending on how hard he was throwing I might decide to toss a donut (for the non baseball junky this is a weight you place on the bat) on my trusty bat or swing the heavy weighted bat which always felt like Paul Bunyan's Axe. I would then time my swing with my newly weighted bat, take off the donut or switch back to my bat then retime my swing. Recent research from Jacob M. Wilson and Abraham L. Miller on the Effects of Various Warm-Up Devices and Rest Period Lengths on Batting Velocity and Acceleration of Intercollegiate Baseball Players shows us that if you use a weighted device in the on deck circle of between 26 to 50 oz you should complete you're warm-up swings as soon as you step into the on-deck circle (1-2 minutes before the start of the at-bat of the player ahead of you). The study showed that after performing a warm-up with a weighted bat, bat velocity peaked after 4 -8 minutes the warm-up. Wilson and Miller also suggest that hitters try to take a few pitches before swinging in an attempt to reach the optimal rest between warm-up and swinging to reach their greatest bat velocity. Wilson and Miller also say that if each batter was to wait for their optimal rest period then they could add to pitchers pitch count in an attempt to get them closer to their total pitch count, which could cause teams to burn through more pitchers in a series or fatigue the current pitcher faster. Pitchers and Catchers may also note that if a hitter does not stop warming up 1-2 minutes before stepping into the batters box his bat velocity will be slower during the first few pitches.
Wilson, Miller, Effects of Various Warm-Up Devices and Rest Period Lengths on Batting Velocity and Acceleration of Intercollegiate Baseball Players; Journal of Strength & Conditioning Research: September 2012- Volume 26 - Issue 9 - p 2317-2323.