Basically I change tyres twice a year, One winter set and one hardpack set, Oh then theres the odd occasion when slicks go on for a road ride. In fact I don't--3 sets of wheels so if it is gloopy in the spring I can put on the Mud set of wheels, or if it I know theres not much mud on the route I am taking on that ride, Then the normal offroad wheels are used.
But then I only ride one area and I know the terrain I am going to meet. I know the tyre that works for me on that terrain so only need one set of offroad tyres.(Fire XC's in 1.8 and Mud Pro's in 1.8 for the gloopy stuff). As I say theres then the Road rides and I use Continental Grand prix's for that. Then of course then there's the Tandem but thats a different story.