1. Personally I do no more than 20 sets per body part. I believe the max for a body part is 30 at the highest end. 12 is sufficient for maintaining and stimulating some growth.
2. Always try and increase the weight that way you are tearing muscle fibers and making new ones grow in their place. Remember you have to break a muscle down to build it up.
3. At least three to four times a week. That way you can at least come in and hit every muscle group. I go 5 but I do a different training style.
4. The rule of thumb for pure training is no more than an hour. The only time I spend longer than an hour is when I train 2 body parts in one training session. I usually end up in there for an hour fifteen or hour thirty max. If you are training with cardio as well I can see being in the gym longer than an hour.