Look around a bit on the forum. We have been discussing this topic ad infinitum here and elsewhere. I don't think you will get an easy answer. The consensus (to the extent that it exists) seems to be that if you sail a lot you should replace it about once a season if you can afford it, but that you can remain reasonably competitive with an older sail.
If I have the money I'll retire a sail from top-level racing when it loses that new 'crispness'. When that happens depends on the conditions I raced in and how well I stored the sail in between.(ie avoiding leaving it in a hot car for days on end)
I have a hierarchy going with sails:
brand new: big, important national or international regattas (it's a feel-good thing; I'm still going to do lousy relative to the hotshots)
lost a little bit of crisp: regional level stuff or really windy national regattas
starting to get blown out but not a complete rag: weekly practice
total complete rag:taking three friends out for a ride or tearing around in the edge of a hurricane or something else ridiculous
My system:
Two sails. One new, one 'old'
New sail used for 3 seasons for summer Sunday racing and the odd open
Old sail used for winter series and everything else (practice, hurricanes, etc)
After 3 seasons new sail becomes new old sail and old old sail gets sold on Ebay for around £40
Cost of new new sail around £400 (-£40 quid from Ebay = £360)
Summer Sundays and race events actually attended over 3 years =around 60
So cost per days racing = £6 ish