This probably is very individual. You can think of two concepts: minimum effective dose versus maximal recoverable dose. It’s a balancing act and sometimes you need one and other times you need the other and most of the times you may not even know the exact point from which a stimulating dose begins.
And you need to think both of those concepts in combination with your interval sessions. Overall, with dedicated training you want to give your body a stimulus, recover, progress and repeat. So in combination with harder interval sessions I would keep the z2 sessions more on the easy side and concentrate on nailing the interval days (as @Helvellyn said). And there are other times where I do those upper end, longish z2 sessions, which feel hard and exhausting in the end, but then I think of those sessions as hard days (and would not put hard interval days on top before being recovered again).
And sometimes there are those young or genetically gifted that can do week after a week, combining hard days and upper z2 days without burning out but I would be very cautious with that approach.