After reading the BMC Care and Maintenance Booklet it seems very much a 'how long is a piece of string' debate. The UIAA Safety page has document downloads on all aspects of climbing equipment. The Essential Rope Info page on Needle Sports website convinced me to downsize back to 50m after a couple of years of lugging 60m around [because it is occasionally handy in the Alps is not a good enough reason]. Anyway after 15 months of : 2 or 3 times a week trad, one alpine summer, no leader falls and a bit of furring - we have retired our Beal Joker 9.1mm.
