I doubt that 18 month figure is official. Thats just what the news sites have been saying in response to the droves of dead TCs out here:
http://en.wikipedia.org/wiki/Time_Capsule_(Apple)#Failures
http://www.tuaw.com/2009/09/11/are-apple-time-capsules-out-of-time/
http://www.pcworld.com/article/173675/users_complain_as_apple_time_capsules_die_young.html
http://www.computerworld.com/s/article/9139367/Dying_Apple_Time_Capsules_spark_complaints
http://www.guardian.co.uk/technology/blog/2009/nov/04/apple-time-capsule-failures-early
http://www.crunchgear.com/2009/09/11/apples-time-capsules-croak-after-18-months/
http://www.geek.com/articles/apple/apple-time-capsules-dying-by-the-droves-after-18-months-20091012/
http://timecapsuledead.org/ reports an average a little longer - 19 months, 20 days.
http://en.wikipedia.org/wiki/Time_Capsule_(Apple)#Failures
http://www.tuaw.com/2009/09/11/are-apple-time-capsules-out-of-time/
http://www.pcworld.com/article/173675/users_complain_as_apple_time_capsules_die_young.html
http://www.computerworld.com/s/article/9139367/Dying_Apple_Time_Capsules_spark_complaints
http://www.guardian.co.uk/technology/blog/2009/nov/04/apple-time-capsule-failures-early
http://www.crunchgear.com/2009/09/11/apples-time-capsules-croak-after-18-months/
http://www.geek.com/articles/apple/apple-time-capsules-dying-by-the-droves-after-18-months-20091012/
http://timecapsuledead.org/ reports an average a little longer - 19 months, 20 days.