Since I spent a lot of time working on it while learning it I didn't really want to have to keep putting a lot more time into it. But I'd hate to lose it.
If it's a piece that stretches your current technical capability (i.e. you're having to play right at the edge of your technique to play it properly), you'll need to do more than just revisit it once a day, at least for a while, to maintain it. Little gremlins creep in easily, and get ingrained.....
I suggest that you take extra time once a week to really practice it (as in isolating the problem spots and practicing them by themselves, extra slowly if required, before reinstating them into the piece) until it's 'perfect' again.
After anything from several weeks to months, you'll find that you don't have to do this quite so often - maybe once every two weeks, or once a month.
It can be time-consuming to maintain such a piece, so make sure it's something you really
want to keep indefinitely. Otherwise, let it go, and you can always pick it up again later on, and you might well find it much easier than you think to regain fluency, after you've had more repertoire under your belt, and your technique is much better etc. Don't forget that easier pieces are much easier to maintain.