Since BMWs are made to last, it's more than likely that you'll be driving yours for a long time.
The engine and transmission of your Bimmer will never let you down, but some other  parts  might  
give in if they've been overworked.

It's the case of the alternator. If you fitted your car with various devices that require a lot of electricity, the alternator might give in after prolonged abuse. Such devices include massive subwoofers along with their amplifiers, various lights, LEDs and almost any other part excepting the ones fitted to your car when it left the assembly line.
Symptoms that your car's alternator might be failing soon include low battery voltage,a squeaking noise when the car needs extra power and last, but not least and probably the most obvious,a warning
light on your dashboard (that's when things get really messy).
So, in order to replace your alternator, you'll need a DIY with step by step instructions. Those can
be found anywhere on-line but you'll also need some tips on how to do it faster, in order to finish
faster (time is money after all).
If you're set and want to do everything by yourself, the guys from M5Board.com, have a dedicated
page here that has some useful tips on how to get through this whole ordeal with the least amount
of headache.