There is no character limit in the builder. How long a title will render depends on where it's being rendered (header, list button), the mobile device, and the font you have applied to your project. (For instance, a phone with a wider screen will show more of the title than a smaller one).
As a frame of reference, on an iPhone display, the page title in the header will show about 18-20 characters (including spaces) before being truncated. For a site list with icons, this is closer to 17. A good target range for page titles on most devices is 12-15 characters.