You can play for free for as long as you want. The amount of time you can play before hitting the free subscription limit depends on your development speed. An average player should reach it after approximately four weeks. Once you hit the limit you won't be able to expand your kingdom anymore, but you'll be able to play as long as you want. There is one caveat to this, If you already have three heroes (and you are not a subscriber to the game) you do have to wait 71 days before you can hire another hero. After 10 weeks of playing all free accounts will have their limit upgraded to four heroes and four cities, every 6 weeks after that another upgrade will be made making so that you can have +1 max heroes and +1 max cities. If you have a subscription to the game this time restriction is removed, and you can have as many heroes as soon as you want.