I want some decent muscle size and strength development, how much time should I be spending in the gym?
The main concept to get your head around is optimum effect isn't going to be due to you spending hours on end in the gym. If that was the answer there would be a whole lot more immensely strong, well muscled people around.
I've been guilty myself for over enthusiastic extreme workouts, along with plenty of people I know - weight training sessions of up to two hours or more, six days a week. This doesn't work, actually you can be heading backwards fast and at worst setting yourself up for injury or illness.
Science along with hardcore bodybuilders like Dorian Yates have shown us that keeping each weight training session short and intense (30 to 45 minutes) is the MOST effective time frame for stimulating muscle growth. The key here is intensity. You must achieve a high degree of intensity, nothing more.
From a volume perspective, six to eight working sets (not including warm up sets) for smaller muscle groups and eight to ten working sets for larger muscle groups is all you need to do. If the muscle has reached the point of complete failure, and you've worked out way beyond the pump sensation, then you've gone too far. You'll actually be doing a lot more harm than good.
When intensely weight training your body will endure up to around 45 minutes until the force of the stress hormone cortisol kicks in, which can be responsible for breaking down muscle tissue. Before you reach that turning point get out of the gym and home for a fast whey protein hit to start rebuilding your muscles.