If a player does not have enough stamina, a serious drop in his performance will be noticed, particularly during the second half of matches. You will notice his drop in performance in his stars rating. The total number of stars shows his average performance during the game, while the number of yellow stars shows his performance at the end of the game. So the more grey stars, the more performance is lost due to low stamina.
You can decide what percentage of your normal training to dedicate to stamina (minimum 5%), the amount needed to maintain a certain level of stamina depends on the age of the players. Stamina training efficiency will drop at higher intensity level, it´s better to train a little every week than a lot now and then.
Midfield Stamina Effect
The following percentage gives the impact of stamina on the midfield rating for a player with the same form/experience/playmaking rating.
The following gives the minimal percentage required to maintain stamina at solid at various ages:
The following link will show you a complete table to obtain certain stamina level, with ages (17-38) and percentages (5%-50%). Those results are based on a study made by Shulo. Data was recolected in 4 federations and global forums of 12 different countries. This is the most reliable study about new changes of stamina.