How long is an extruder's/nozzle's life?
This is highly dependent upon the type and quality of filament you use.

Exotic materials such as wood, metalic, or otherwise infused plastics can potentially wear out the inside of the nozzle and at the very least leave deposits of composite material in the nozzle. This can lead to more frequent or irreversible clogs. You may also notice your prints becoming "stringy-er" (sorry for lack of vocabulary at the moment) as the volume of your nozzle reduces with more deposits sitting in the basin of the nozzle, therefore leading to lighter extrusions against what your slicer is expecting.

Lower quality materials (common ABS/PLA) can also have an effect on your nozzles life span. So can good quality filament, don't get me wrong. However, lower quality materials have a tendency to go through much more drastic material changes during the printing process than better quality filaments. For instance, a lower quality PLA may not be 100% PLA. It could have 10% Nylon, 5% ABS, 0.1% operator's BandAid, and 83% PLA. Well, the 17% of the filament that isn't PLA has the potential to clog the nozzle just as easily as the exotic filaments. This is why it's so important to verify the quality of your source of filaments.

Some signs that your nozzle may be wearing:


It simply clogs more frequently
The printed part looks "stringy" or "bubbly". The "stringy" effect is that the extruder is not extruding at a fast enough rate for the speed of the motion, which is common if your extruder is about to clog, your filament is crappy quality, or your slicing settings are invalid for your machine. The "bubbly" effect can be from an overheated extruder or another sign that your nozzle is about clogged.
Your filament begins curling around the nozzle as it initially comes out. Typically there is a burr or something that catches the filament on its way out that you can simply take a small file to grind it off, but sometimes it may be because there is the build-up of deposits towards one side of the basin.