Yeah, with hair that long (
especially if you're in your teens/20s), you want to wash it every day. It varies from person to person, but rule of thumb is that it'll start getting noticeably greasy ~12-24 hours after the last wash. And if you're going to keep it long, you owe it to yourself to keep it clean as well, otherwise it'll itch, help cause acne, and otherwise fuck with your skin and complexion.
My advice is to find a good all-in-one shampoo and conditioner, pour out a circle ~1 inch/2.5-3cm in diameter every day when you shower. I remember switching over to that from some organic shitshow back when I was sixteen or seventeen, went from greasy every ~10h and dandruff to no dandruff and greasy every ~30h. Also, if you keep it longish and don't already:
trim those split ends. Get in the habit of trimming regularly: it helps keep your hair healthy and neat, and will actually make it grow faster (allegedly).
Granted, it's supposed to damage your hair to wash with shampoo more than 3-4x per week, but when you're young you basically have a choice between hair that's dry and clean vs. keeping to that. Your hair will naturally become less and less greasy as you age, so you probably won't have to do it forever... but yeah. Clean hair means everything it touches is going to be cleaner as well, and that's a big plus.
Uh. Shit. Hair care's important, yo, but I should probably make this less derail-y, so have some dog.