I used to spend 45 minutes in the gym, but I did 6 different lifts
Try to alternate what muscles you work so you can gain faster and keep your muscles balanced; add a variety of different lifts. I've gained a crazy 8 pounds in 3 weeks(with the help of meat and protein drinks) through doing this during my first month of lifting:
Monday: lower body
Tuesday: upper body
Wednesday: cardio/flexibility/agility training (don't need the gym for this)
Thursday: upper
Friday: lower
as for adding weight and how often, I myself added on 5 pounds every week or two to my bench lifts and lat pulldowns, but it's really just a matter of how much you're actually progressing and what feels right to you