The longest recorded sermon of William Branham available at Voice of God recordings is "The Spoken Word is the Original Seed". This sermon is 377 minutes (6 hours, 17 minutes). It was preached on a Sunday at the Branham Tabernacle on March 18, 1962. The preaching was split into two parts with approximately 90 minutes of intermission between halves. Keep in mind though that this is only the preaching. There was a song service, announcements, and likely Sunday School prior to the recording of the morning service, and likely some worship before the preaching of the second half.
How many have attended longer? What's the longest you have ever sat in church in a single service? In a single day?
I have personally attended a service that lasted from 6 PM until approximately 2 AM, so about 8 hours. The morning of that service there was an additional approximately three hours. That was about 11 hours total within a 24-hour period sitting in church.
Here's the even bigger question: what is the benefit of these ridiculously long services?
Who does preaching in this kind of marathon actually benefit?