Interesting to hear some guys not re-using it, I thought that was the whole point. Mind you it's such good bang for buck I can see why they wouldn't bother.
I use an old 3l juice bottle, with 4.5ml of starsan. Then I pour some of that into a 300ml spray bottle. The rest of the 3l bottle gets poured into (cleaned) fermenters, shaken, let sit for a couple of minutes then poured back. The (massive) foam gets poured out onto my concrete brewcave floor....well the foam that'll come out...the bit that won't I just leave in. Don't fear the foam!
I am deliberately careless with pouring it back once used, which means I probably go through those 3 litres in a couple of months through natural attrition. If I ever see it going cloudy (rare), I'll piff it and start again.
I've been using the same few fermenters and bottles for well over a year now, and no infections to date. I use commercial sodium perc for cleaning everything, so the starsan is doing nothing but sanitising, I'm sure that would contribute to the long life of my starsan solution.