Hello all, been awhile.

I wanted people's opinions on how frequently you should practice a song once you have learned it in order to keep it mastered. As of now, I have learned a number of (relatively) difficult cover songs in order to increase my technique and want to move on to writing new original repertoire while perfecting my existing original repertoire and developing other musical skills.

As long as it takes.

Seriously, nobody else can give you any advice on this as its wholly dependent on your abilities and learning processes. It also depends on WHAT you want to play. If you're bored with a song and want to learn some new stuff there's little point practicing the one you don't fancy playing any more.
Actually called Mark!

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Just to clarify, I wasn't asking how long to practice a song until I learn it. This is a discussion about how frequently you should practice a song you can already play to a high level of proficiency in order to retain it.
That doesn't really change the answer, it's still pretty much "as often as you need to". This is one of those "how long is a piece of string" questions; I can think of songs I haven't played for about 3-4 months that I could pick up the guitar and play perfectly tomorrow but by the same token I can think of some songs I played at a gig less than a week ago that I'm not sure I could remember now.
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Can you play the song right now?

Yes? Good. Practice a different one.

No? Practice the song.