Variety: 'Better Call Saul' Team on the Season Finale SPOILERS
Odenkirk and Gould talk about the episode and the future. It's a good read. I think from a lot I've read from Gilligan and Gould, they really don't have a planned timeline, as if to say character X has to transform by this episode, or this guy has to show up by this episode.
But Gilligan said about Breaking Bad that he made it six seasons (or was it an extended 5th for 5 1/2 seasons?) because shows pretty much run their course by then. When you think about it though, this show could go on much longer because Odenkirk is multiple characters. They spent 4 seasons on Jimmy McGill/Slippin Jimmy. With tiny bits of Saul teased. You could do say four more seasons on Saul, his eventual interactions with Mike, Nacho, Lalo, where Kim is going? Then you could do x amount of seasons on Gene because that is a whole different world that opens up to brand new characters.
Honestly, this series could go on for many many more seasons because there is so much story to tell because the guy is so many different characters at different points of time.