1. When you have enough $$$ in bank account.
2. When you are no longer satisfied with the sound produced by the current cartridge.
3. When hear your friend (or anybody) bragging about a new cartridge and how good it is.
4. After finding out the needle is worn out (inspecting with at least 30x magnifier glass or microscope).
5. When you have just found out a real deal going on ebay (or agon) and would not want to miss the deal.
Actually, those were most of the reasons why I replaced the cartridge/needle. 5-3-1-2-4 is the order (the most frequent/compelling to the least frequent/compelling reason) that have occurred to me. I guess I have bought about 15 cartridges over the last 10 years, from Grado ($30) to Clearaudio ($250).