For the amount of tire wear that I do in a year, here's what seems to work:

1. Ride one season on a set of tires.
2. When winter comes, keep using those tires on the trainer, and on occasional outside rides when the weather is nice enough.
3. When spring comes again (now), the rear tire will be pretty worn, from both ouside use, and flattened a bit from the trainer. Take off this tire and discard it. Put a new tire on the front of the bike, then put the used front tire (which will have much less wear) on the back.
4. Ride one season on that set.
5. Repeat.

So a tire's life (again, for my amount of riding) is: One season on the front of the bike, then one season on the back of the bike (including winter on the trainer), then trash.... one new tire each spring.