I have managed to put on just over 5kg of mass (hopefully most of it muscle but there is probably a bit of fat in there too!) in about 20 weeks.
Actually it took about 3 and half months and my weight has kinda settled as I have been tailing it down a bit as going the gym all the time, doing a busy job and having a baby daughter is knackering.
Bit of background, i am (was) a skinny dude. Whatever I ate made no difference and whatever workouts I did made no difference. My starting weight was 69kg, now I weigh 75.4kg
How did I do it? Its actually fairly simple. heavy intense compound workouts along with eating a VAST amount of food.
checkout
Bodybuilding.com - The Future Of Bodybuilding! Huge Bodybuilding Site.
Great website with tons of really helpful advice and videos
The compound exercises are exercises that use lots of different muscle groups at the same time. For example, squats, deadlifts, pull ups, dips, bench press. Its a very efficient way of working out as you can get a fully body workout in a short time. For it to work well you have to do low but heavy reps if you want to gain mass. I was using a 5x5 rep set.
I limited cardio to essentially warming up and nothing else as I tend to burn calories fast and it was preventing me from gaining weight.
You need to be eating 6 times per day. Thats right, SIX meals. Of course they don't need to be 3 courses each but the idea is to keep your body supplied with a decent amount of carbs and protein during the day. You need to be eating A LOT of protein as well (dependent on your weight). I took whey protein powder drinks as it was proving extremely hard to eat the amount of protein required to gain muscle.
An example of my daily diet was
Breakfast - Large bowel porridge + 2 boiled eggs
Mid morning snack - Protein drink + oat cakes
Lunch - Tin of tuna with pasta
Mid afternoon - 2 portions of fruit + Ryvita crispbread
Workout
Protein drink + 2 bananas post workout
Dinner - Whatever me and the wife chose to make that evening
1/2 pot cottage cheese before bed
So as you can see its a crapload of food. Literally, I was having to go for a dump 3 times a day with all that food! And the whey protein drinks make me fart something terrrible - just thought I'd warn you as it doesn't say that on the tin!
If you want I can post my starting workout which is based on 3 workouts a week. Its simple and easy to remember.