The wedding I went to a few months ago was a total of about 13-14 hours (getting ready at about 9AM, photos, ceremony followed by cocktail hour, reception and dancing, and then a send off at about 10PM) as they had the entire venue rented out for the entire day.
Others (in facebook wedding groups im in) are saying that 8 hours is too much and that i don't need that much time (it's what we want and we are not doing the traditional wedding exactly, sister and I are performing a dance for everyone that no one except the photographer I may hire, knows about, we are also playing clips of home videos from us being younger and such, and much more) and I have a timeline built up already, and 8 hours is still not necessarily enough time for everything we have planned, but I was afraid that having anymore hours from what we originally want is ridiculous.
We have about 6.5 hours from our ceremony start time the end of our bar service. We have to be out of the venue 30 minutes later which is fine because we aren't responsible for clean up.
My reception venue (since I'm having my ceremony by a local pond) we have access to all day and the day before. They're new and allow weddings to come in the day before to set up and we technically have the venue until 11:59pm.