It's not normal; 18-24 months is typical between rounds. If you are gaining significant non-monetary benefits (eg learning a lot from much smarter people, real

It's not normal; 18-24 months is typical between rounds. If you are gaining significant non-monetary benefits (eg learning a lot from much smarter people, really interesting problems), there might be a reason to stay, but if you are being underpaid and there is no reason to expect it to change, I would get the hell out.