Poland went a long time until 2010. Finland's been going without a top prize winner ever since it started, so I think that one takes the biscuit.

Afaik, it is Turkey. the show started in 2000 and until 2019 they finally have a winner. Another version is Vietnam, which have been on air for 17 years without a single top prize winner.

Ecuador hasn't had a Top Prize winner since November of 2002.

Hovewer, The Top Prize Winner is John Carpenter, who answered a correct answer in 1999 on Who Wants to Be a Millionaire (USA)