excellent response from Rudi.
I'd only add that with a good looking, second hand tensioned leather saddle it may have been treated with a range of products (other than Brooks Proofide) which may or may not affect the longevity of the leather; or if it was excessively treated with Proofide, that too may have damaged the fibre strength.
I had a Brooks saddle given to me that was 12 months old. Too many treatments of Proofide and it was unusable.
Easy for me to put a new cover on.