The average ceremony lasts between 7 and 9 minutes; that is from the bride’s entrance to the signing of documents.
This all depends of course on what is in there ceremony content but generally there is a standard format that most couples wish to follow. Time can be extended by extra readings, a music recital, an added special ceremony or by additional material to further personalize the day.
My advice is to put what you feel comfortable with. Remember it’s your day.