A lot depends on how well you are able to use your fat stores as fuel - in theory if you are well enough adapted you have enough fat to fuel you for many hours.
Personally by trial and error, and at a steady endurance training pace (for me around aerobic threshold) I've found a little over 2 hrs to be a reasonable time. I suspect I could go longer but I try to be pre-emptive and start fuelling early rather than bonk at 3hrs plus.