My most profitable ever session was a 27 hour marathon last year in a 'social' game.... at the end of it I was still fresh as a daisy (well.. compared to the others)... it was like watching the scene in The Hustler where Fats is fresh and Fast Eddie is absolutely wrecked. We were all getting drunk during this time too which helped to loosen up a few opponents purse strings.
A sign that I'm tilting on-line is usually that I stop playing poker and head for the roullette
I agree that if you've been playing for a good session and your losing to stop and start afresh though... no point carrying on while probably tilting and your mind aint quite there. If you're winning though I say carry on and on and on until the tide turns.