I am in this exact position so I hopefully can give you some insight...
I took lesson from the age of 14ish to 18...played daily and was IMHO a pretty good player, by no means would I think I was great or anything, but I was getting to the point where I didn't have to think about what I was doing I could just go and let the spirits take me wherever.
From the time I started college until now(14 years) I have played very infrequently. For some extended periods no playing whatsoever.
I am making an effort to relearn now and have committed myself to playing again. That being said, I can still pick up a guitar and remember most of the technique and chords and whatnot, actually getting my hands to do them though is a different story. Also, I have to really think about what I am doing now and my memory of songs is greatly diminished. My ear is still very good, I can still basically quickly tune a guitar by ear if you gave me a E or A pitchfork. I think that if I didn't make any effort to relearn I would stay at the level I am at now for the rest of my life.
But I would say overall ability wise I would compare myself to someone who started playing 6 months ago.
I would also say that my ability to stand and stare at a wall full of guitars at GC with my mouth watering is as strong as ever...
In some respects though, relearning is a far more frustrating experience then I remember learning the first time to be. My sessions now are peppered with a lot of "Sh!t, I used to be able to do that!".
Back to the original question though, I don't think someone who seriously played could ever completely lose it to the point where you have the same ability as someone who has never played, I think you will always have the ability to pick up a guitar and play something and not sound terrible, even if it's just vamping on an A chord and playing Magic Bus until you pass out...