When you lose it.
Seriously, I still use a line on my largemouth bass/panfish rod that hasn't left the reel in 20+ years. I just replaced the DT line on my 4 wt with a WF line so I could shoot bigger streamers better, but the DT line is 5 years old and would have served me well into the next century if I didn't like spending money on fishing crap
. I imagine I'll be using the WF line until one of us falls apart. I'm betting the line is going to win.