Endomorph = the fat one = 'typically' sumo wrestler type of build
Ectomorpth = small wrists 'typically' the jockey guy (guy who rides on a racing horse)
Mesomorpth = the stud = 'typically' the 100m 6" type of sprinter big build and muscled and lean
15-20lbs IS A HELL OF A LOT OF WEIGHT. You ever picked up 2lbs bag of sugar? Well that is HEAVY to be mainly all muscle. It will be hard to pack on that amount unless you have starved yourself or have never done weights and ate very badly.
I say a 3-5yr or so to properly pack on 20lbs of solid muscle or to have muscle and fat and to lean down properly.
"I know to increase calories quite a bit (not sure of what numbers exactly) and to focus on some heavy core lifts."
That is IT.
It isn't hard to pack on muscle over time.
EAT a LOT OF FOOD every dam day. Eat every 3hrs and you will pack on weight.
CYCLE your training. 4 months of the year work heavy weigths and low volume (no more then 25reps in per exercise). Then the next 2 months do high rep work 3 sets of 12-15rep stuff. SHORT REST INBETWEEN no more then 60seconds. You won't be lifting heavy but you will get a lot of volume and keep the intensity high through the short rests. Then get back on to the heavy stuff again. Then maybe eat less food and up your cardio for 2 months or so and just lift whatever you want. Eat less and lean up a bit.
Then start again for next year.
Bulking = eat anything and everything you can get a hold of. Eat chicken, turkey, fish, tuna, some beef, anything you can get a hold of. Go easy on the skin of the stuff.
BUT I gurantee you one thing. YOU PROBABLY WON'T BE EATING ENOUGH and then will wonder why your not packing on the muscle. OR you will see you are packing on some fat then will eat less. WELL you will pack on some fat when you bulk. No one packs muscle without some fat. If they do it staying lean it takes longer and some can't do it staying lean. Some can but you'd probably be some what bigger already if it was you.
Bottom line: Eat a lot, lift and cycle your lifiting intensities and volume, do some cardio and more cardio when you cut, DON'T pay attention to the fat you put on when you are bulking. Cut every year.
If you do that you put on the muscle.
Me I'll pack on the muscle mass I need slowly as I'm not bothered. More interested in my OL performance. But I know by the time I am 26-33 I'll be amazing with my lifting and be stacked.