400 hours is a bit much with most cartridges I would say. My Denon 103R had pretty much blossomed by 50 hours. I've heard of extreme examples of around 200 hours. Given that you can break in most cartridges on the Cardas Sweep Record in about 2-4 hours, and that it is valuable for regular maintenance beyond that, I view it as a must have product for anyone owning a turntable.
From my perspective, it would be pretty depressing to go 400 hours and then figure out something was drastically wrong when you can do it in 3 or 4.
From my perspective, it would be pretty depressing to go 400 hours and then figure out something was drastically wrong when you can do it in 3 or 4.