Aluminium does have a fatigue life but in most cases if your not a pro racer or a tourer doing tens of thousands of kilometers a year your not liable to wear out the frame before every thing else is knackered but .............
if you use an a-frame type cycle trainer a lot you can clap out an ali frame in a very short time because your subjecting it to loads it was not designed to withstand and the more hours a session and the more force you apply then the faster it´ll happen .
As a general rule the lighter a component then the shorter it´s working life and this holds true of frames as well ( mind you I´ve seen ex-racing frames from ONCE being sold on and lasting for years after so .............. ) ( ONCE used Giant frames which tended to be amongst the lighter frames in the peloton )