Is it bad to "hold it"—as in, not go pee—for long periods of time? If so, how long is too long?

Yes. Holding in your urine for long periods of time may predispose to urinary tract infections. Doing this chronically can even increase your risk for incontinence later in life. Try to urinate when you first feel the urge.
Can be. If you chronically hold your urine for many hours, you increase the risk of getting a bladder infection, and if you really hold it too long, your bladder might stretch enough so that you can't empty your bladder for a few days, and might need a catheter. To shrink it. This is more likely if you are older, than it you are a young adult.