I'm not certain how long they last but my initial impression is not
good (both sensors failed on my 2007 GT within two years :( ) But...
I'm not sure that they died due to low battery because while I did get
a fault code it was generic (TPM sensors not reporting) and not the
specific low battery alert. I believe that the specified battery life
is supposed to be something on the order of 3-5 years. And of course
instead of the $50 generic sensors available for many cars the BMW
units cost about $150 each, and are BMW dealer-only due to a deal with
Schrader (why am I not surprised? ;-) Luckily physical replacement
(during a tire change) is no big deal, but does require the new
sensor's serial code to be programmed into the TPM control unit.
Having to go to a dealer for that is just adding insult to injury.
Hopefully my original units had some sort of manufacturing fault and
the replacements will last longer.