How often can I change my registered IP?

There are two slots, primary and secondary. Any change to a slot (adding, editing or deleting an IP) locks that slot for 7 days, so you can't put a new IP in it until the lock ends. Each slot has its own 7-day cycle, so a change in one doesn't affect the other.

You can always delete an IP, even during the lock, if you need to cut off access immediately. Deleting doesn't reset the clock, though: the slot stays locked until the original 7 days are up.