MC Cartridge break-in?

10 hours minimum, i'd say. you should notice the biggest difference after that minimal time, and then the time increments really start to spread out. ie like, 50 hours and so on. i can't imagine it needing more break in any further than 150 or so before it is finished.

either way, though, the first 10 hours are the biggest.

I think Lazarus is right on for most cartridges. However, I have owned a few that still sounded pretty bad even at 30 hours of break in. The other thing to keep in mind is cartridge loading as well as VTA. Both can play a major roll in soundstage and openness. The nominal cartridge loading is just that--"nominal" and almost never optimal. You have to experiment a while to get the best sound. However, get at least 10 or more hours on the cartridge, because any changes you make during the biggest changes during break in will likely be changed again.

Is there a "best way" to break in a cartridge?

I don't really know. Before I had a Goldmund arm, which has a repeat, so I could use an old decrepid LP, I didn't like, I simply had to exercise patience and waited until the music began to bloom.

Try the Cardas Burn In disc. I'm not sure I got the name exactly right, but it has white noise tracks which are actually one continuous groove which can play indefinately. These discs are pressed on special hard, durable vinyl for repeated use. The disc also has many other diagnostic tracks which are quite useful.