That is one of the ways in which the writters of the Real Football game app plans to earn money from it. When you are said to be short of stamina, they will give you two options.
1. To either buy the stamina with real money or
2. To rather wait an X number of days (literal days)
Each one of the options comes with a sacrifies. The first one gets you expended financially (to their compesation) and the second one forces you to keep away from playing the interesting game for the specified number of days!