Over the years I have found that eight or nine hours does a good job of documenting most weddings. Some smaller weddings might not take that long. Some weddings, however, might need more hours of coverage based on the interval between the end of the reception and the start of the getting ready process.
My recommendation is to work backwards from the final moment you want me there. Let’s take a typical wedding day scenario and break it down. A reception might go from 7pm to 11pm with a cocktail hour beginning at 6pm. That’s five hours. Perhaps the ceremony is at 4pm and ends at 4:30pm. That leaves an hour and a half for post-ceremony portraits and transportation to the reception venue. You’ll probably want to arrive 30 minutes prior to the ceremony, and perhaps it’s a 20 minute drive to the ceremony from where you are getting ready. That’s pretty much 3pm to 11pm, which is eight hours. Add one hour for “Getting Ready” photos, and you’re at nine hours.
Now, there are variations on this which must be considered. If you want coverage from earlier in the day, you’ll either need to add an hour or two to the package, or have me leave the reception a little early. It depends on what is most important to you. If I know ahead of time that I’ll be leaving the reception one hour prior to its conclusion, I make sure the venue and DJ are both aware of it and I get all of the “activities” scheduled prior to departure time.
Basically, work backwards. Do you want the entire reception documented? If so, work backwards from the end of the reception time and figure out what time you want me to start.