This really varies depending on the person. I can get just a bit more than 6 months before I notice it takes longer to do the same part of the face. I also notice a big difference between shaving when the humidity is high or low. The one thing I do that really helps is to keep the blades clean using some sort of cleaning solution. Ir makes a big difference even though you can't see it.
As much as I want to get 18 months, I find that 12 months is the best I can get before I get tired of the time it is taking and I swap blades. The first Norelco I had in the 1960s lasted forever, I never changed blades until 1980 but those days are over.