- do you experience this and how do you deal with that ?
Yes, I maintain a repertoire and I also always keep learning new pieces. For me, what happens is that I eventually become either so bored or so thoroughly familiar (kind of the same thing) with a piece that it's basically dropped from maintenance and this opens up space in the rotation. However, I have a set of caveats: always make it a point to have a clean record of the phrasing, articulation, dynamics, and fingerings, so if one wishes to pick up the piece again, they don't have to reinvent the wheel, as it is very frustrating to have to waste time on things already thought about and having expended a great deal of effort on. (On the other hand, if it seemed some aspect of the piece never really clicked, especially the fingerings, it is almost always better to start afresh).
- is there a moment when the piece do not need to be maintained anymore ? (maybe after numerous relearning ?) ; a moment I could have not came across since now since I'm rather young with this method ?
Not that I've experienced (see above). It's more a voluntary choice than anything else. Sometimes I overlook a piece in the rotation and suddenly remember, "Oh yeah, there's that Bach invention I haven't played in a month."
- do you spend time maintaining pieces ?
Yes, around 50% +/- of my practice time, some days more, some days a great deal less. Usually, the pieces most in need of maintenance will order themselves to be rehearsed.

Disclaimer: I am not a concert pianist or piano teacher, and I do not earn my living playing the piano. I play the piano for my own enjoyment and anyone who cares to listen. But I like having built a repertoire because, for me, the difference between reading a piece and actually "knowing" it is night and day.
