Com base na conversa de Luke com Lars sobre sua saída, acho que é temporadas planetárias, com uma colheita anual:
LUKE: I think those new droids are going to work out fine. In fact, I, uh, was also thinking about our agreement about my staying on another season. And if these new droids do work out, I want to transmit my application to the Academy this year.
OWEN: You mean the next semester before harvest?
LUKE: Sure, there're more than enough droids.
OWEN: Harvest is when I need you the most. Only one more season. This year we'll make enough on the harvest so I'll be able to hire some more hands. And then you can go to the Academy next year.
OWEN: You must understand I need you here, Luke.
LUKE: But it's a whole 'nother year.
OWEN: Look, it's only one more season.
LUKE: Yeah, that's what you said last year when Biggs and Tank left.
(Quoting from Revised Fourth Draft, January 15, 1976; but the dialog hasn't changed much from that to the final release IIRC)