You have to know what sound you are going for. Tweaking for days on end can be fun and you can learn a lot from doing it but it doesn't lend itself to finishing anything.
If the guitars don't sound the way you want them to, why didn't you record them differently? Did you know what you wanted them to sound like, has your expectation of the guitar sound changed? Or is there no plan for the song and you're fiddling with it until something sounds "right?" That isn't mixing, its playing with your DAW: home recordists get the 2 confused all the time.
I don't fiddle much anymore unless I'm getting to know a new plugin or piece of gear. Even then the goal is to get the know the gear and not finish a mix. When I'm wiriting and recording a song its a different process.
I usually record a demo kind of quick and dirty- its well recorded in case I get some incredible performance but its not intended to be the final recording. This is where I work out all the parts and arrangements. While tracking I already have some idea where I want it to go, but that usually shifts as parts are added or removed and the creativity flows. I'll do some heavy tweaking here to wrestle the sounds around and see if the ideas are working. Eventually the "final" form of the demo emerges and I know what each instrument's part and sound is going to be.
Then I make an mp3 of the mix and listen to it for a while to see if it really works. Give it a few days to see what rubs me the wrong way and what I really like about the arrangement. If it still sounds good I get ready to record the "real" song.
This is much more painstaking. I know what sounds I'm looking for so I record them that way. I know the arrangement so I play the parts straight through- and practice them so that I can play them well before I even start recording. The hardest part is keeping the feeling going and keeping the performance alive- so parts of the demo that do have that energy are sometimes brought in to help keep the energy flowing as long as the tempo is the same.
Mixing is then fairly simple. And it might be done when the song sounds like a kickin' version of the demo. There are usually things that come up- great ideas that you didn't have before- that shift things around but that's the fun part of mixing. Then you have to be honest with yourself and know when you've done the best you can for the song at hand.
If its a tune that I intend to release on my next CD then I usually don't mix it myself. I'll do a reference mix for the mixing engineer, but by that point I am way too saturated in my own clever ideas to be objective about the song. I schedule some time in the studio, fly over and its done (whether I think its perfect or not) when the scheduled time is up.
And even though I say "whether I think its perfect or not" it invariably sounds better than if I had stayed in my studio tweaking it. Thanks to the joys of the DAW, fed ex, and fast internet connections (as well as waning pro studio demand) there are really good mixing engineers out there that will mix your tunes for fairly cheap.
If that's not in the cards, though (and for most of us it isn't) there is the Clinic.