I honestly find that it varies based upon usage and how they are handled, stored, and cleaned. Overall though, my MAC brushes tend to have a shorter peak to their life than many of my others.
brushes with proper care can last for YEARS!! one of the MAC trainers and my lovely MA have had their brushes for over 8 years and they look brand new! i mean the handles show wear but the bristles are fine!
I had a 213 e/s brush from mac that I bought in 1994. I finally replaced it this year with the 239, but I still can use the 213. I just found it wasn't packing on the colour like I wanted it to. It still works just great for a wash. I brush off excess colour onto a damp towel after every use. Then once a month I use a brush cleaner. My brushes look great.
my 03 holiday brushes are still in pretty good condition, and those are just holiday ones... not even the long-handled brushes. but i guess it really does depend on how much you use them (i use them quite often). with proper care, they should be able to last for a long time