There is no set page length for a film treatment. That being said, the reasons why a Screenwriter would craft a film treatment can impact its length.
If a Screenwriter is crafting a treatment to help clarify the script’s story for themselves, it might be 10 to 12 pages in length. If they’re writing it to pass along to others who might be interested in the script but not wanting to read all 100 pages of it, the film treatment might be two to three pages. If it’s a studio or production company that is seriously interested in producing the script, a treatment could be upwards of 20 pages or more to provide enough story details for the studio or production company to decide if they want to make the film.