I used to go to a big, fancy gym in Richmond that had a multi-coloured disco ball in the spin room. Good times.
This is where I met Kevin. He informed me that he worked his upper body in the morning, went home for lunch, then returned to work his lower body and do his cardio in the afternoon. Presumably, he then had dinner and filled out his exercise journal for fun.
Unless you want to end up as creaky and dull as an old treadmill, do NOT be a Kevin.
Here's what you need to know:

Don't work the same muscle group to fatigue within 48 hours - let the muscles repair after resistance training!
Combine resistance (weight training) with cardio throughout the week
Three to four times a week is ideal and do things you enjoy even if they're tough!
Your workout should last between 45 to 90 minutes including warm-up and cool-down
Your fitness and ability to burn off calories depends on how much incidental activity you do - not just your workout (ie. walking, fidgeting, taking the stairs...)
How often and how intensely you exercise is dependent on your goals and your current health and fitness
If you are experiencing injuries or illness on a regular basis, your exercise program needs reviewing! A skilled personal trainer can assess your routine and work with you to make sure your time and effort is paying off
Change it up. Roughly every 6 weeks, try changing your routine - whether it's a new set of weights exercises or a new class, this is essential for optimum physical benefits and for the mental challenge
Never, never, never go to the gym more than once a day. If you are, there is more at issue than your fitness. You need to consider why you are spending so much time in the gym because it's not necessary, or healthy*! *Unless you are an elite athlete with a program of fitness and nutrition that enables you to exercise safely and effectively with adequate recovery periods