Regardless of the "time to fail", if a failure does occur, you may want to focus a little more on the time it takes to get back up and running.
According to Michael Peterson of Strategic Research Corporation, "MTBF ratings for tape libraries are no longer a true measurement of reliability when building mission-critical capable network storage solutions. The proof of reliability comes with the supplier's reputation, warranty, and service policies. If you accept the fact that failures happen, the true measurement of reliability is system availability."
Read Pierre Dorion's answer to this question.
This was first published in September 2004