Prem Ratan Dhan Payo is not yet released,
Even this movie not premiered in any film festival also.
But its Wikipedia article showing running time. How is it possible?
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Likely because this is details that movie theaters need to know in order to properly schedule screening times. They know this weeks ahead of the premier because not knowing would result in very sloppy handling at the theaters.
Any movie to be released will have a final cut made weeks to months ahead of release, and locked in a run time when it is submitted to the CBFC/MPAA for a rating. They can't edit to shorten or lengthen without resubmitting.
Additionally, run time is not a specifically special or important thing to hide from the general public.
Wikipedia just gathers its information from other sources some of which are mentioned in the references. It is likely that one of those references have the running time mentioned. This information is indeed disclosed by the producers/distributors, because of the reasons mentioned by cde's answer.
An example : Ron Howard's film In the Heart of the Sea is scheduled for release on 11 December, but if you check its Wikipedia page, the running time is mentioned with a small link over there. If you go to that link it will take you to the page of British Board of film classification, where the running time is mentioned and that is where Wikipedia picked it up from.