Solos evolve over time, until you just have to say "that's enough". I'm playing Blues lately, so I'll take my examples from that genre.

Blues just naturally involves playing a lot of covers of other artists' work. Usually, the axemen take turns stepping out to do their bit towards adding something fresh to a song that's been around for years. Your own material gets the same treatment as the band works it out. The trick is to keep it just familiar enough fhat the audience can follow you, and fresh enough that they'll want to. Eventually, a point is reached where its 'just right', and further messin' with it just takes away from it.

Check out "Blues at Sunrise" from Albert King's "Live Wire/Blues Power" album. Thats a lot of solo work. Its been polished and worked on for a lot of years, starting from an amalgam of at least two much older songs. My point is, once he got it to where he was satisfied with it, he let it be. It stands on its own as one of the blues masterpieces. Even when Albert and SRV did their duet on it, they didn't change it much, just took turns at it.

At some point, a song reaches maturity, and its time to invest your creativity on other material. You can always come back to it when its been around long enough to get 'stale'.