Two lines of thinking on this one:
1 - As per a recent Cool Tips from Charles Staley: "I prefer whole-body workouts, training 3-4 days per week. But, even with whole body plans, each workout should still be slightly upper or lower-body dominant. Also, if you go three times a week, don't be afraid of long workouts: 90 to 120 minutes is fine, particularly if you're taking longer rests from big movements like squats and pulls."
It's important to note, though, that he's talking about full body workouts. A 2-hour workout dedicated to chest and abs hasn't been the "in" thing to do since 1980-something.
2 - It's generally agreed on that Growth Hormone, Testosterone, and a host of other homrmonal goodies are maximized when resistance training is kept to 60-75 minutes, tops. So, ideally, I'd shoot to keep things closer to an hour or so. That can be hard to do sometimes, especially if you're lifting in a group.
At 18, you could probably benefit from just poking a weight plate with a stick. But, there are better ways than that. For now, I'd say that anything that has you lifting consistently is good. We can find a better way after you've established some kind of base.