I have a very short attention-span when it comes to working out. Cardio ends up boring; I’m too impatient to wait the 1-2 minutes between sets for weights; I get bored with more than 20 min of either.

So my current goal is to go to the gym 3 hours a week. Half of that devoted to cardio, half to weights.

My current workout:
10 min cardio warmup (elliptical)
10 min weights (3 different muscle groups, 3 sets of 10. Different muscle groups means I can skip much of the between set waiting, as I cycle through the exercises)
5 min cardio (to kill the boredom, get my heart rate up)
10 min weights (3 different exercises again)
5 min cardio
10 min weights (same deal)
10 min cardio (includes cool-down)

A workout in an hour, half cardio, half weights, with enough variation to keep me motivated.

Also, I time it with music. I went and built a playlist of 12 songs that are about 5 minutes long. Around song 6-8 I put in some really upbeat stuff so I don’t feel like leaving.
(That’s my biggest problem, the “eh, I’ve done 25 minutes of this, that’s enough”)