What percentage of movie theatre revenue is from pre-film commercials?

Just curious. I know that concessions is the bulk of theatre revenue and that box office is only a small portion. But yesterday I saw a movie and noted that there were 10 minutes of commercials even before the trailers started. These commercials are probably the only video ads now-a-days that still have an entirely captive audience.

So has advertising become a major part of the movie theatre business plan?