It is at least 25 minutes, but maybe more depending on stops the taxi makes. The taxi system there is a bit strange, they are vans mostly and they try to fill them up before they leave and they charge a per person fare. It is all regulated though and they have to charge the same between the companies. I think was either 8.50 or 10.50 I will offer some advice, get out EARLY! Last year I missed the ferry by just a few minutes. It leaves on the hour. I am assuming you are going to St. John.
Best to you,