Hi uk_climber,
The question on how long is “long enough” really depends on your goal objective, but also on the total volume you are able to be accomplishing each week. Generally speaking, i rarely have athletes train over 5 hrs in any given session unless they are completing a specific prep workout (such as running a 50km trail run/race as preparation for a goal event); using these long durations require a much greater recovery period to follow, and can hamper a consistent training load.
Without knowing your goal objective I hesitate to say you should add more volume, but I would suggest you look to the other days and sessions in your training week/microcycle, and consider whether you could get more value in your training by adding quality elsewhere, such as a moderate intensity session or muscular endurance.
Cheers,
Sam.