That might be a record for most questions ever asked in one post.
How do you know when to take off and build again?
I go by looks yes. There is no set standard. I know some people that go strictly by weight. But I don't like that. I set a general weight goal, but if I get to soft before that, I will cut the bulk short. Or, if I reach that point and am still looking lean, I would extend it.
Is it just all based on looking in the mirror?
For me, mainly. It makes the most sense IMO.
How long is a typical diet session in this situation?
This varies from person to person. As I said before, none of us lose and gain weight at the same rate. For me, I tend to shed fat rather easily. The ratio is usually roughly 2:1, so I diet half as long as I bulk. Personally, I've found 4 months of bulking and 2 months of cutting to be my sweet spot, but this is certainly not written in stone.
Can you cut the build and diet times in half and not get as drastic of a change? I mean, could you have caught yourself at 200 pounds but then dieted for half as long, and keep your abs showing and keep the bodyweight down to a minimum?
It seems like you are working under the assumption that there is a set way of doing this. There isn't. The basic laws of energy balance apply. Eat over what your body needs and you will grow. Eat under and you will shrink. Grow to a point where you are comfortable with, then shrink to lose the fat that accompanied the new growth. Easy as that.
Some people like the slow approach, trying to stay very lean year round. I used to do that but found that my muscle growth was too slow for my liking. Now I bulk using large chunges of time... 4+ months. It feels like I can build momentum doing this.
I have seen people bulk for a month, cut the next. I have seen people bulk for 3 weeks and cut for 1. I have even seen people eat surplus for 3 days and deficit for 1.
Personally, the shorter of a time ratio you use, I think the less muscle mass you grow on a time weighted basis. But you probably would stay leaner for longer periods of time too since you are never extending your body into long time continuums of over-feeding.
Follow me?
Is there a minimum time that it takes for building?
No. Some people grow like weeds, others like bricks. I am in the middle of that someplace.