How long did the Iguanodon?

If you mean "How long did the Iguanodon exist for?" the answer is that they lived for about a million years or so, between 126 and 125 million years ago.

If you mean "How long was the Iguanodon?" the answer is that Iguanodon grew to be 33 feet long as an adult, but especially large individuals were up to 43 feet long.