Seven (7) hours. Between William Muldoon and William Miller in the original Madison Square Garden on 3-23-1880. Match started at 8:47 and lasted until 4:00 AM. Muldoon also had a match the next year with Clarence Whistler which lasted from 8:40 to around 3:30 AM. They were allowed ten minute breaks after each hour. The owner of the venue turned off the gaslights which ended the match as the men got ready for the seventh hour.
In modern times Bruno Sammartino wrestled Pedro Morales is a 65:05 draw in 1972.
Also Chris Hero won a 2/3 fall match over C.M. Punk on 2-7-2003 in Clarksville, IN for IWA Mid South which went 92:15.
I forget the details but in 1987 Inoki supposedly wrestled Masa Saito for two hours in the middle of a jungle (really)