Originally Posted by
PeteHski
The real limit is how much training volume you can actually tolerate, even if you have the spare time. Most average people start to struggle with more than about 12-14 hours per week of mixed intensity riding, but it varies a lot individually. It's very easy to over-train and then hit a plateau or even start losing fitness. Structured plans at the way to manage both volume and intensity and are usually goal oriented.
Agreed, provided the conversation is about any kind of fitness returns at all on the investment of training hours as opposed to increasing returns on that investment which typically only occur when starting from an untrained state. The OP seemed only to be interested in the latter.