Some will say that they've never had to replace the bladder in their fuel cells, but in the racing world, 5 to 7 years is the recommended life of the bladder.
People may go longer, but they won't last forever.
http://www.atlinc.com/faqs_racing.html
"Q How long does a fuel cell bladder last?
A Most ATL racing cells are warranteed for 5 years, which is the legal life span established by most racing organizations.
Q Does FIA allow for recertification?
A Yes, under FIA rules, an approved bladder* may be returned, in its 5th year, to its manufacturer for inspection, and if warranted, up to 2 years of life extension will be granted."