This game is so fun to play and doesn't require you to play all the time to unlock and upgrade characters. I usually play and complete the missions, and then play again once the missions are refreshed. Also the game gives free chests every once in a while and it basically takes no time to claim them, so if you don't have much time to play you can still unlock and level up characters. Also, new characters keep getting added and there's almost always an event so there's always something new.