How do I know when to change cartdridges?

I'm new to analog, having obtained a Rega P3-24 a couple of years ago. I'm wondering how I will know when it's time for a new cartridge. My concern is that in such a mechanical system, the wear-out will be gradual and thus hard to detect. Which is fine in a way: if I don't know it has degraded, then why should I care?
I have a Dynavector 10x5 now, which I believe is a high output moving coil. I've seen several people post that unless your budget is much higher, you should stick to moving magnet. If that's the case, what would be a good moving magnet cartridge to get for this setup?
If it makes a difference, I'm using a Dynavector phono preamp which is switchable for MC or MM and Audio Research tube amplification. And I doubt I would get approval from CINCHouse to spend over $1,000 for a cartridge.
Thanks for any guidance.
You shouldn't care. If it's degraded you will notice that most of your records would sound dull. Once you change the cartrige or retip it you'll have a feeling of new strings on the guitar.
It's not about age, it's about hours of playing records, of course. Many think that after 800-1000 hours you better replace or retip the cartridge or risk damaging the vinyl. Personally, I follow this recomendation; and I don't buy expensive cartridges.
Age is a consideration as the suspension will stiffen with time. This is the primary concern when buying a "new" or even a lightly used cartridge that is 15+ years old.

However, if you play vinyl on a regular basis, the stylus will probably need attention before the suspension.

I use a microscope and ocasionly inspect the stylus for wear, so far I have had many years of use from my ortofon VMS20E, with no wear to worry about. clean vynil correct setup equals years of use, abuse causes much more damage than wear and tear ever did. Get yourself a small, good microscope, check out a spare cartridge for practice. What you are looking for is two small `flat spots' where the stylus touches the sides of the groove, if you see these develope, it's time for change. You can also check out new styli before use, a chiped tip trashes your precious record INSTANTLY!