I believe Harry Stone had over 200 bouts without ever being stopped in his career. I did a thread on it a while back.
Missed the bit about start of the career. Sugar Ray Robinson did 136 fights without KO defeat before the Maxim fight?
Stone holds the all-time record for most fights during a career without ever being stopped - but here we're looking for a world champion.
I know Benny Valgar has 200+ fights without a stoppage, but he wasn't a champ and I think he had a DQ. Pancho Villa also had a DQ, so not him either I think.
Pep was stopped for the first time in his 137th fight. Leonard was stopped several times early in his career.