This page of the Stanley Parable Ultra Deluxe game guide contains information about the number of endings and the length of the title.
Number of endings
Stanley Parable Ultra Deluxe has a total of 27 endings, including 9 brand new. Also, after unlocking new content, you will get a bucket. By taking it with you on the adventures, you will discover new variations of the 20 endings. Sometimes they differ from the original ones to such an extent that they can be considered new, full-fledged endings.
The length of the game largely depends on how much you want to discover. The title can be finished in about 5 hours with the most endings unlocked. However, this time can drastically increase if you want to see all the endings and unlock all the achievements. One achievement is about running the game for 24 hours (in one go). On the other hand, the Art ending can only be unlocked after playing the game for 4 hours (as long as you don't make a mistake and will have to start all over again).