I just started to walk them every morning. During the week it is about 20 mins. On the weekend we do just over a mile, so how ever long that takes, plus I stop them often and carry them a little bit too (so about 40 mins.)
They also get to run in the backyard at times as well.
They are really enjoying the morning walks, I am trying to keep them NOT so bored during the day, as Roxy was starting to do bad things