I've been using Brooks saddles going back almost 5 decades. I no longer do because I got out of the habit years ago, and have come to prefer the easy care that leather plastic saddles offer.

But original Brooks Pro, lasted well over 50,000 miles, and has never needed retensioning. I chalk this up to using enough oil to keep water out (It saw lots of rain), but not enough to soften the leather. Also, the fastest way to kill a leather saddle is to ride it when it's soggy.