I'd say 2-3 season each year, 1 will be increadibly boring, 4 too much for user with less time.

I'd say 3 seasons (each 4 months) will be the best, it will let the player that want to get on floor 100 to reach so and letthe player that have fun in the beginning play more.

I usually play 1-2 months and stop, after that i get bored.