It depends.
I think more and more managers realize that stamina is actually the "3rd" defensive skill, hence very important. Your players irrespectively whether national team or club team will play worst usually in the second half or foul more with low stamina.
Having said that, I think it is self explanatory
In our U21 program, there is a strict guide stamina must be trained even if you need to sacrifice a skill training. From 18 to 21, you can manage this I believe relatively well. There should not be player in the roster with lower than 5 stamina.
Sz.