The Spring season consists of 12 total days. Once you complete a day you can see your time for that level in Free Mode on the main menu. This season will take roughly 1-2hrs depending on how good you are at puzzles and your problem solving skills.
You can use wildseeds from this season onwards throughout the rest of the game but you only get four of them in total. You can use a wildseed from the pause menu to skip a level you're stuck on, you should use these wisely for when you really need them in the later seasons.