A frequently asked question at this stage of the competition is: 'Are you strict about the 15 pages rule?'
The answer is 'no', but we're really tough on scripts that last longer than 15 minutes. It's the only way to guarantee a level playing field for all our writers.
How do you know how long your script runs? Read it aloud. Preferably with friends. Time it. Ask for their (honest) feedback.
The one-page-of-script-equals-one-minute 'rule' is a rule of thumb. In other words it's not terribly accurate. As Dan O'Bannon points out in his Guide to Screenplay Structure the film of Castaway runs to 143 minutes. The script is 92 pages.
We don't know what font you're using, how you've laid your script out etc, so we can't tell you in advance whether 15 pages is suitable or not.
Another advantage of reading aloud with friends is that you'll find out which speeches are redundant, which characters are underdeveloped, which lines are difficult to say, which jokes don't land etc, etc. And you can discover all this for the price of a bottle of wine and some pizza. It's worth it.