Geoff, I assume some of your heads are now out of warranty so no concerns? I have 2 heads now saying out of warranty but can't see any difference! How do we know when it's time?
Is there a way to determine how many clogged nozzles have been remapped to spares? I believe this is what determines the life of the head ... at some point all the spare nozzles of one color are used, so no way to compensate for more clogged nozzles.
If you can't be down for a day or two, you would have to keep one on hand, because I think what will happen is it will suddenly tell you to replace the head, and until you do you may not be able to print. But being out of a warranty period would certainly not be an indication of needing replaced.