Bilbo aged noticeably after parting with the ring. Gollum had it for something like 500 years? You'd think he'd instantly turn into dust after losing it. As far as I can tell, there's no difference in his appearance at all in The Hobbit to when he appears in LOTR.
The longer duration for Gollum probably meant that the ring's influences were stronger on him and took longer to undo.
That's a good question. He's without it for 60 years, and he doesn't deteriorate quickly like Bilbo does, even though one would expect it to be even more accelerated since he's 600 years old. Huh.