I've been wondering about break in; I've noticed many stylus(es) have the thinnest coating of glue/epoxy/resin or whatever it is that seals the diamond and post to the hole in the cantilever. If any of it finds its way to the tip during the manufacturing process, it could possibly affect the sound a little bit. I suspect this wears off, and if it is on the thick side (sometimes it is) it can take a little longer or perhaps just slide off after an hour or so. If this is the case, there might be no hard and fast rule that it is one hour or ten to break in a stylus. But the other aspect of this is gaining a slight bit of polish from the grooves of a well played but good record, and this might be regarded as proper break in as well.
I have seen some well played, good quality styli come by that have a gritty silica-colored coating of dirt, oil, record cleaning agents, whatever kind of stylus-crete you can imagine. It can be so hard and thick that there is an appearance of a missing or worn diamond at the end of the post. I must chip this off under a microscope using a good sewing needle, sharpened toothpick, tiny brush and alcohol to see what is underneath; usually it is a very nice stylus. Can only imagine the sound was not quite as bright as it otherwise should have been. Typically takes 10-15 min to clean one like this, sometimes longer. Detailing a cartridge can take an hour or longer for me to do a decent job.