You can still get up even when the seatbelt sign is on. One of the FAs might remind you to be careful and that you're getting up at your own risk. I've been on many transcons where the seatbelt sign is never turned off, but there's zero chance I'm going to be able to go 5+ hours without needing to use a restroom. My rule of thumb when traveling is to do what you when you can (e.g., go to the bathroom when able), not when you have to. If it's calm air, even with the seatbelt sign on, I tend to use the restroom at that time just in case it does eventually get turbulent. (The only time I don't recommend doing this is when you're clearly on descent, during the now defunct 30 min within DCA rule, etc.)