Can anybody post his opinion of how long before a player will be worn out in each level of stamina?
It depends on how many minutes a player plays in a game, week and season, as well as what their stamina level and sublevel is. All players lose stamina little by little, but it is slow enough that we usually don't see it until they drop a level.
And I know that their is also a disadvantage of a high stamina so my question is, whats the most ideal level? and whats the right level so that a player can go full blast in 48 minutes.
My ideal level of stamina for my players is mediocre or average. That way, they won't be too high, but they'll be OK if they drop down a level. As for what the right level is for 48 minutes, there isn't a perfect answer. Strong-proficient is the best bet if you need to play someone the whole game, but they don't need to always have high stamina. For example, I have a trainee with atrocious stamina. Even though his stamina is at the lowest, he is still capable of playing 48 minutes.