For on the road repairs I always replace the tube with a new one, but then repair the tube when I get home.

If I am fixing it at home it depends on my mood.

I usually use tubes for up to three punctures (ie I'll patch it twice but throw it out when it gets the third puncture) but yesterday when I got a flat on my commuter bike I discovered my spare already had four patches on it - some people might go even longer but for me thats time for a new one.