I think Caterham to Tattenham Corner is probably the longest single zone journey outside of zones 1-2. I’m not sure how many people actually want to do it. When I last checked this many years ago it was posssible within about an hour, but timetables have probably changed since then.
TfL do increase the time allowed for certain journeys where it isn’t possible to complete it in the standard time allowed. If they get reports of problems then they will likely look at increasing the time allowed, which is why I’m not convinced that many people actually make the journey, if any.
If you do make the journey and it takes longer than 70 minutes then you’ll be charged two incomplete journeys. When you call the helpdesk they will refund all but the single zone charge, and it may kickstart a review.