Benz Micro needle life?

How many hours of playback should I be able to get out of a Benz Micro cartridge before it needs retipped? How can I tell if mine is due for a retipping? Also is it better to buy a new cartridge instead of retipping? Will retipping change the sound any?
That's like asking how many miles you can get out of a set of tires. Depends on how they're used. An upper-range Benz, properly mounted and aligned, cared for and cleaned properly, will last significantly longer before re-tipping than an entry-level one because it has a finer stylus. An obvious sign of wear is audible mistracking, but by that point you may already be damaging your records.

The only sure way to tell is to take it to your dealer and have him put it under a microscope. I've had some cartridges where the styli were worn after a few hundred hours; I've had others that lasted thousands. It doesn't sound like you're comfortable yet with this aspect of analog so I would take it in for examination after a few hundred hours. Based on the visible wear (or lack thereof) your dealer should be able to ballpark the lifespan.

Retipping, if done by the manufacturer, shouldn't change the performance at all, just as replacing your car's tires with the exact same make and model wouldn't. You can also go to aftermarket companies and have them fit an improved stylus and cantilever, which may be worth it. Retipping is almost always going to be more cost effective than replacing the cartridge. On the lower end of the price scale, you may also be able to trade in your cartridge for a new one and receive a credit for the returned model.
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As Elizabeth says, best case is to wash all your LPs, but life is too short. I read a thread on the gon a few years back, when the responder said he washed every record, every time he played. Seriously impressive if true, or a textbook definition of OCD, I am not sure which.