Q: how long should I bulk up before I go to the calorie-deficit stage? Or does this depend on how much/how fast I want to lose the weight after?

Also, I am a 19 year old female. What is 'bulking' up for me now--gaining how much weight? I already weigh 155 from strength training for two years, but never devoted time specifically to bulking up

A: It's really completely up to you how much weight you want to gain in any one bulk period. The success of the bulk is usually judged by the amount of new muscle gain compared to new fat gain. You want, of course, more muscle and less fat though most likely you'll end up gaining some of both. Different people will have different tolerances for how much muscle-vs-fat they can accept, so in the end it's up to you to decide how much to gain and/or when to stop.

If say, 10 pounds into your bulk you find that you haven't gained any noticeable amount of fat (and assuming you want to gain more muscle) you can certainly continue the bulk. On the other hand it may be that after just 5 new pounds you notice that your old pants are getting tight at the waist -- then you may want to consider stopping the bulk and maybe lose some weight back to cut the fat. But again, only you can make the decision.

As a woman it will be tougher for you to gain muscle as quickly as men can, so to minimize fat gains you'll want to gain weight as slowly as possible. My usual recommendation for men is to try and gain about one pound every couple of weeks. You should consider that the upper limit on weight gain speed and I would advise to go even slower.

The other thing to consider is that you mentioned that you've naturally been able to gain weight to 155 pounds over two years without doing anything special -- if you started at 140 or less then you have already been gaining weight at near-optimal speed! If that's the case, you may want to just keep on doing what you've been doing. Good luck!