It's all about how much time you want to spend on the game.
You can farm Green rest packs which may take around 20mins a day to help train your players.
I've managed in 8 months to get around 400 green packs and 2300 tokens without spending any money but by checking regular for apps to download to gain tokens.
Some of the gambling apps will give you 600-700 tokens just for depositing £10.
I've managed to build a squad of 7 stars in that time.