Each person works at a difference pace and has different time available to study. In my case, I started Season 1 back in March and it’s just gone mid July and I’m onto Season 5. The thing that slowed me down was learning long lists of vocab. I’ve had a lot of free time this year though so If I was more busy, I’d probably be around Season 3 or 4.
The main thing is being able to recall what you’ve already learnt.
As for the dip, I haven’t really felt discouraged learning more grammar, vocab and kanji. If anything, It makes me more determined to get closer to my goal.