It will depend on the ride and the riders.

Example: yesterday me and a Friend rode 30 miles to a fast group ride. On the way there at 20-22 MPH on flats we spent 10-15 minutes up front 10 for me and 15 for him. the group ride does 22 miles out at an easy pace around 21 MPH in a double pace-line the turns are all about 2-3 minutes. When we turn around it is hammer-fest 25-28 MPH with pulls (depending on ability) of 5 seconds to 2 minutes. After the ride we rode back at slower speed 18-20 MPH and traded pull more frequently say every 2-5 minutes.

So as you can see there is no right amount of time. It will all depend on the rider and the group, mainly the rider. So pull as long as you feel comfortable, but as we say don't hog all the wind!