What was Beethoven's longest piece of music?

The longest symphony ever written is Alexander Scriabin's "Preparation for the Final Mystery", realised posthumously by Alexander Nemtin who worked on it for 26 years and finished it in 1999. It lasts close to 3 hours. In the first stages of its conception and before his untimely death Scriabin imagined it to last for 7 days, accompanied by a colored light show, incense and dance.

Formally it was assumed, Havergal Brian's symphony "Gothic" was the longest lasting about 2 hours. This one you can find listed in the "Guinness Book of Records".