It depends on how dedicated you are. There are a couple of good books out there. One is called Brawn by Stuart McRobert. I also read Beyond Brawn but the original is better IMHO. Another good book that's more condensed is Super Squats:How to gain 30lbs of muscle in 6 weeks by Randall Strossen. I tried and gained about 5lbs every two weeks till I stopped after two months. About 2/3 was muscle and the rest was fat.

The fat is inevitable unless you are genetically blessed or are doing steroids. BUT knowning how to trim down is in your blood so don't fret the extra fat. Just go until you feel you want to trim down the fat. That's what I did. Aerobic exercise is great. Run. Do jumping jacks while watching commercials. Skip rope. Then once your fat is down to a comfortable level, hit the squats again. Oh, also eat lots of GOOD food. Your appetite SHOULD soar with the squats IF you do them right so that shouldn't be an issue although it could be...so make sure you eat lots throughout the day.

Here's a sample of what I would eat. Morning, 2 eggs, 2 sausage, toast and pint of milk. Mid morning snack, apple, pint of milk. Lunch, sandwich, pint of milk. Afternoon snack, 1/2 sandwich, pint of milk. A good dinner, TWO pints of milk. Fruit snack before bed, 2 pints of milk. Yup, that's right 8 pints a day or a whole gallon. I tell ya my poop was white! But I was determined. Are you?

Are you ready to load the bar up (once you know proper form), psych up your mind, and squat heavy! squat deep! squat for 20 reps! I did 20 rep breathing squats. That is you do one rep of the squat and when you come up you breath in 3 DEEP breaths. Then you keep doing that till you get to rep 20. Now, if you've put enough weight on, the deep breathing will come naturally around rep 12-13. Your body can't help but want to breath deeply! By rep 14-15 your mind is fighting your body to give up! Will it? Can you make it to 20? If you're not asking this question by rep 15 you didn't put enough weight on the bar. By rep 16-17 you are dying! BUT your legs are MUCH more powerful than you think! They can do it! It's your body. Command it! Once you hit 18 you're almost there. Your mind knows it and will bust out the last 2 reps! You are done! Go die now. You'll really feel like you are about to!

The good thing is that you only have to do the squats about once a week. Then 2-3 times a week work out the rest of your body with COMPOUND exercises. That is exercises that work out major muscle groups. Exercises like pushups, pullups, bench press, overhead press, rows, pull downs are all great.

Really a great place to get more information on gaining weight through weight lifting is hardgainer.com forums. Get one or both of the above books.