This is only from my own personal experience but I would NEVER play a show without fresh strings. I see guys spraying WD40 on them here to stop corrosion ( I guess) but c'mon if you are playing to an audience for $$, then you should be able to fork out the $6 for fresh strings no?
In the studio? again, fresh strings befor every session.
In the bedroom or jamming I would leave them on until my fingers (or the guitar fretboard) cant take it anymore.
Strings in this climate have a very short life expectancy anyway. Lucky they are cheap!