Ideally, you're spending your time in the gym working on compound lifts (instead of targeting specific muscle groups), using that time efficiently and taking appropriate time in between sessions to recover.
In 1999, I was a rookie with the Detroit Tigers. After a game, I strode confidently into the Kansas City Royals' weight room, rested the weight bar on my shoulders and settled in for some heavy squats. I rocked my hips back and dropped into my form, feeling totally in control. On the way back up, I got stuck. I leaned forward and the bar spilled over my neck. More than 500 pounds went violently crashing to the floor and members of both teams turned to look.
Luckily, nothing was injured that day other than my pride. It could have been much worse. I was over-trained; I had an intense weight training session the day before and was just coming off playing nine innings. "More is better" was my mantra, but I was naïve.
From my teenage years on, more days of lifting was merrier. I split up my routines and broke them into muscle groups in an effort to spend more days in the gym.
During this time, my routine would be broken down to one muscle group a day. Chest on Mondays, back on Tuesdays, legs on Wednesdays, etc.; you feel me, the classic bodybuilder workout. The theory here is that you break each muscle down individually through isolation.
I certainly had my share of success with this routine, but it was due to my consistency, not my efficiency. I was spending 11 hours in the gym a week, but I was working harder, not necessarily smarter.
Now, I spend 4 hours a week getting after it (a little over an hour, three times a week.) Instead of isolating specific muscle groups, I focus on compound lifts that work several muscle groups at a time. I fill those extra hours reading a good book or sharing a meal with my loved ones instead.
Research has suggested that 4-6 repetitions of 4-6 sets, increasing weight on each successive set, resulted in the largest gains as the weeks and months pass. I do five sets of five repetitions each, building up to my working weight.
I focus on lifts with the most impact - alternating days of squats, overhead presses, deadlifts with sessions of squats, bench presses, bent over rows.