I was doing either 100k (which is like 4 years) or 5-6 years, for the first one, then shorter after that. That was one the ones that I could just drain and fill, figured, keeping it freshened up was good enough, just drain the radiator and refill. Now I think I need a vacuum to refill and I haven't looked into that complexity just yet.
I also now have to pay to get rid of coolant (used to be free) so that has put a damper on this front. But I'd be pretty happy with a D&F every 50k I think, 5yr max, if I could.