How long can puppies be kept in a play pen?

Small puppies need to go out relieve themselves every two or so hours. So unless you want the dog running around in its own excrement - you can't leave them for more than a couple hours. Many puppies will also EAT their feces if you leave them locked up with them.

If are you talking about setting up a play pen in the yard for the puppy to get fresh air? Then you need to supervise them 100% of he time they are outdoors.

If people are at work or school all day - it will be IMPOSSIBLE to successfully housebreak a puppy.