You don't have to log in every day to play this game but if you don't log in to your club for a long time you will be fired and your club will be freed to get another manager.
If you have attached an e-mail address to your account you will stay in the game for 5 weeks (35 days) after the last time you logged in. If you haven't you will be removed after 2 weeks.
If you don't log in to the game for more than 2 weeks your assistant will slowly start using experience points to train your players.
If you have a paid account you will not be fired from your club before your V.I.P. status expires.