I track the ffishing hours I put on each of my lines. I think in terms of the usable taper, as opposed to the whole line. So for a double taper, I have gotten 100 (Masterline Chalkstream) to several hundred (Cortland 444) before I have to turn them around. I have an old AirFlo that looks more durable than the 444s. Some of the newer models strike me also as maybe more long lasting too, but I don't have enough hours on them to know yet.
I've played with using the back end of WF lines, that is, the running line, for close in work for my 1-3 wt rods. Of course I had to mess with adjusting leader butts and the leaders themselves to provide some semblance of an overall taper. When the end wears down, snip it off and reattach the leader. This way, I figure I can get thousands of hours out of the line.
I haven't put enough hours on any of my silks to know their longevities, but I'm expecting those to go thousands of hours.
Since I have a couple of hundred lines left, I need to keep working real hard at using them up. Tough work, but somebody has to do it.