I'm replacing mine four times per year, so every three months. My jobs are never heavy cuts because my machine rigidity is low. Light cuts, but long run times, sometimes up to 14 hours per day. I've found the key to longer bearing life is to keep the RPM's low. I rarely run anything faster than 15K RPM. I'm using a Bosch 1617EVS. Be advised that the lower bearing is not even rated for the maximum RPM the router can achieve. Most routers will eat bearings if you run them flat out max RPM.
I think the reason I get good bearing life is because of the SuperPID. When I just ran it with the stock speed control, it was hard on bearings.