My understanding is that the trade off risk of injury vs physical adaptations gained is around the 2hr mark, 1h45 for heavier runners over 80kg iirc. Runs longer than that would be as specific prep for longer races such as a marathon.
I had a similar question a year or two back on this, the first problem in direct comparison with cycling is that there is essentially zero injury risk compared to running Secondly, again iirc, there are few studies on injury risk in cycling to cycling volume.
On the point of physical adaptations alone, not vs risk, rationally there must be some point at which your body has heard the message that you need to adapt and it doesn’t need another hour on the bike to start signalling, but I don’t think there are studies that have identified it.
Personally, as an Ironman triathlete I just listen to my legs as I feel I know when I’ve hit that point on endurance rides that it’s going to affect the quality of my next workout or my next day of training.
However Adaptive Training is having a fair stab at determining it, I had Hoffman last Friday. 4.5h at 0.65 followed by a 1h run which I was a bit apprehensive about but AT called achievable and sure enough I was fine for both.
Because long riding is not like running, I think that point of diminishing returns extends with your aerobic endurance. If you do 2h rides in winter, 3h in spring, you’ll need 4hrs to push the envelop in summer, for example.