It depends. I'm currently playing with a full epic lifespan, and have the seasons set for 14 days each. But my sim is currently living in Isla Paradiso, which is tropical, so I have only spring and summer enabled, with each being 28 days. With NRaas Tempest, I've set the spring weather to be only slightly cooler than summer, and have all four holidays enabled. I also have given children and teens summers off from school, with NRaas Story Progression.
Normally, I play shorter lifespans, and have each season set for 7 days. But I have decided I like playing longer lifespans, and when I am finished with the current game, I'm thinking of trying something like 300 to 400 days, and having summer and winter each last 14 days, with spring and fall lasting 7 days each.