I have been DJing for over 12 years now and have been a TSP user for around 4 years.
I have used the carts and stylus that came with my first “beginner” decks and then purchased a pair of Ortofon Concorde Scratch needles (full cart and stylus) when I got my 2010’s. Since that day, over 10 years ago, I have only ever replaced the stylus, never the cart.
If they are working properly carts never have to be replaced. just the styli when they become worn or when the suspension becomes non-responsive (common when you use a lot of tone arm weight and do a lot of scratching)
The only time a cart may need replacing is if through damage or constant removing of styli the contacts inside the cart and the stylus become worn and have a poor connection. However this is really rare. And you’d know because you’d have things like audio drop outs, static or only one channel coming in.
If your needles work, and you don’t have issues,no need to change them. It all depends in how often they get used, how rough on them you are, how clean your wax is, etc. Good rule of thumb, if your tunes don’t sound all distorted, you don’t need to change them. Just look at the point, and if it’s not mangled, you’re probably ok.