I'd let it go until it starts slipping. Once you feel it start to slip, you'll still have quite a bit of time until it's completely gone.
You can also check the clutch material itself. On the transmission there is a rubber plug on the bellhousing (or whatever it's called on a manual transmission). On my 2.3L 4-cylinder, 5-speed, it was on the lower left side of the bellhousing if I remember correctly. You should be able to see the edge of the clutch plate there. I don't know what the thickness is supposed to be when it's new, or if there is a recommended thickness at which to replace the clutch. Maybe someone else can shed some light on this?
For what it's worth, my neighbor still has the original clutch in his '92 Camry with 240,000 miles. I know there are a lot of variables that determine clutch life, but you may have more material left than you think.
2003 Ford Ranger | FX4 Level II | 5-Speed Manual
A few minor mods... nothing cool enough to mention.