I've always found that stamina levels of proficient or greater mean a player can play 40+ mins without noticeable change in performance.
Actually, the highest stamina can go is prominent (10), and even at that point, the player is guaranteed to have a stamina drop soon after getting to prominent stamina, since it has a sublevel of .00, meaning it will most likely drop the next week. Some players with respectable stamina can play 40+ minutes a game without a big change.