How long untill i can let my new cat outside?

we just got a new cat from someone hes not old but not a kitten hes always been an indoor outdoor cat and we dont know how long we should keep him inside. And i know some people think its wrong to let cats outside but isnt it wrong to keep them inside and my cats are both over 5 years old and they are indoor outdoor cats. i think it cruel to keep a cat inside all the time.