My CTBs are 15 years old. Liners still look good. My left boot does have a dime-sized hole worn through the lining layer in the heel area. But, I never noticed it until I just did a visual inspection before responding to this post.
'09 R12GS, '05 GSX-R1000
Iron Butt # 4999, WERA/CCS EX # 666 (currently, expired), AMA Life Member
Racer, Drummer, Bassist, Software Engineer