There's quite some very long ambient drones stuff by Roach, Rich and others I'm sure... and the average Klaus Schulze track is like 30 minutes (of which some are very good, but most is just endless synth noodling).
But I'd like to mention Manuel Göttsching - E2-E4 (59:34, recorded 1981)
No endless ambient drones, it's rhythmic, repetitive, minimal and very hypnotic. It will drive you nuts, but it's good anyway.