This page says 20+ years. Keep in mind its usually much less in aquariums. I'm not to thriled about the page though because they recommend a minimum 29 gallon tank and I found that even my 6" rainbow shark was crowded in an understocked 30 gallon. For a rope fish I'd think 75+ gallons is more ideal.

http://www.tropicalresources.net/TR_...topic.php?t=13