A “worn” mixing tube is like a worn tool bit: it is difficult to say when a mixing tube is fully worn, but as it wears, it becomes a less effective cutting tool. For precision work, a new mixing tube performs better than a used one. How long a mixing tube lasts depends on a number of factors. Numbers from 20 to 80 hours are fairly typical, although it is possible that they may wear faster, or last longer, depending on circumstances.
Recycling mixing tubes
It is possible to recycle mixing tubes by drilling it out using Wire EDM to increase the bore size. Typically, a mixing tube has a diameter of 0.030″ (0.76 mm). Once the mixing tube is about 75% worn, you can have it bored out to 0.040″ (1.0 mm). The new tube will not cut quite as fast, or as precisely, but you have significantly extended its life. You will have to use an EDM to make the bore, but it can be done, although the nozzle cannot be completely worn out for this to work.