As long as is required. 

On averge I would take 15-20 hours with each couple to prepare and perform their ceremony.

This includes

• meetings with the couple - anywhere between 4-8 times depending on the couples requirements
  
• emails - often many dozens over a period of time
  
• phone calls - often numerous
• writing the ceremony draft
  
• changes to the ceremony - this can often take anywhere from 15 minutes to 2 hours depending on the amount of changes required by the couple.
  
• paperwork - preparing the legal paperwork
  
• on site rehearsal  - this can often take longer than the ceremony itself
  
• the wedding day - this can often take anywhere from 2 hours minimum up to 4-6 hours including travel time.  (I always arrive at least 1/2 hour before the expected commencement time)