I turn if off all the time, especially when using big apps like After Effects, Nuke, Photoshop, Final Cut Pro, Maya, and others. So normally about once a day, but it depends on what kind of work I am doing and how intensive it is.
Personally, I shut mine down at night, or when I won't be using it for several hours. I'm just kinda weird about it, and I really don't mind the minute or so to boot. I don't really have a good reason for it, but that's how I do it. Plus I don't buy that shutting down and booting once a day is bad for it.
The only time I shutoff my laptops is when I know I won't be using them for a week or two. Otherwise they are never off. Probably not the best for the battery, but I don't think it makes that much of a difference, especially with Apple's batteries.
As for battery, I have a 3 year old Black Macbook with a battery that lasts 2 and a half hours.
As for the 5 year old White Macbook, the battery lasts just about 1 and a half hours.
This of course fluctuates depending on how task heavy your usage is.
I have yet to experience, or have heard of a Macbook battery completely dying. I think it also depends on how much you use your laptop without connection. I can't exactly remember, but the battery has enough juice for 200-500 charges (that is, charging after completely exhausting the battery life). That is why you should plug it in whenever you have the chance to.
I basically let mine run all the time except when I need to 1) take it with me or 2) will be leaving it behind for more than a few days. No problems, and battery still registers 100% health.