[time-nuts] Rubidium standard

[Glenn Little WB4UIV]

I was told by a Technical Support Engineer from Symmetricom Global 
Services that
"The typical life span is ~10 years for these Rubidium Time Bases".

This is in response to my request for information on a Ball/Efratom PTB-100.

Is this a typical life span of a rubidium standard?

Do some standards last longer than others?

What are the symptoms of a failing rubidium bulb?

Thanks
73
Glenn
WB4UIV