NL I've heard you should have 20x max buy-ins. Max buy-in is usually 100 BBs, so that's 2k BBs. So once you have 2k BBs for the next level you can move up.
Although this plan is contingent on you being a winner at the stakes you move up to. I haven't heard the number for moving down, but I'd say if you lose 1/4th of your BR you should move down. So if you're playing at 5c/10c you should have at least $200. Say you hit $500 and are properly rolled for 10c/25c now (.25*2,000 = 500). I'd play until 375, go back down to 5c/10c until I hit $500 again and try again. Hopefully you never have to go down, and I just made the 1/4 rule up (although that's what I'll probably do), but the 20x max buy-in is definitely an established number. Although it's bendable too. If you absolutely cannot afford to go broke, use 25x or 30x the max buy-in and if you want to make money quicker at more risk you could use a lower number. But if you want to be reasonably sure that you won't lose your BR and aren't playing for your sole income, 20x the max buy-in is probably a good number to use when moving up.