Unfortunately, not too long. The entire Eddystone Physical Web URL must be no greater than 17 bytes. That said, common prefixes (i.e., “http://“, https://“, etc.) compress down to one byte as do common endings (“.com/“, “.com”, “.org”) etc.
Give the restriction on length, in many cases it will be necessary to use a URL shortener (such as Bitly or TinyUrl). Fortunately, Google's proxy server traverses many of these sites and lists the title and url of the final page.