It's a question I get asked for many different reasons "how long is the ceremony?"
Generally speaking most venues plan 30 minutes for the ceremony (unless your in a church with your pastor then they can often be longer).
I work within that 30 minute window. I plan for 20 minutes, this includes the time walking up and down the aisle so my speaking time is usually 17 ish minutes. I plan for a 20 minute wedding ceremony because something usually happens and we run a few minutes late. It could be those last few guests arriving, a sudden bathroom need or a lost shoe! Either way, allowing a 10 minute window helps so no one feels rushed or anxious about starting right on time.
Now, I do make every effort to start right on time so that the rest of your wedding day time line stays in tact, but rest assured you will get married and everything will fall into place even if we start a little late.