I agree with the above - it varies. Varies with who you are, how old you are, what you're trying to do, where you are in your training cycle. I've found that failure is the best teacher, success not so much. Experiment and see what happens after a few months. Disaster? Success? Watch your HRV and HRs. Everybody's different.

I've found that an hour of Z4, distributed throughout the zone, with 20' of Z5 per week, and with a good bit of Z2 and Z3, not so much Z1, done for a few months, will allow me to do most anything I want to do - have fun with the group, do 400k in the mountains, event rides, whatever. Not racing success, just fun. Racing success is another story entirely, one I've not pursued, being lucky to not have sufficient talent for that, and thus avoiding many issues over the decades and now still at it..