It depends a lot on what you print.
Example: a material loaded with glass fiber will wear out the nozzle faster than a material loaded with carbon fiber.
For PLA/PETG/ABS material, a brass nozzle has an average life of about 800 to 1000 hours of printing. However, this can also vary depending on the speeds used and the temperatures. It is therefore difficult to give a simple lifespan.
As the nozzle (or thermal barrier) fatigues, filament jams during printing become more and more frequent. The quality of the print decreases (edges are less and less sharp for example, and defects appear), and more and more prints fail.
You can follow these procedures for a “nozzle change“.