Ideally, you want to choose an itinerary that helps you and your group get the most out of walking the Overland Track. Set a walking pace and itinerary that lets you do all the things you want to do. Do you like a relaxed pace, walking big days, exploring side trips, early morning photography, or swimming in every lake? Try to avoid rushing your time to make it fit something else, but life happens so make it work the best it can. Remember to walk your own walk, there is no right or wrong pace. 
The longer your trip the more food you will need to carry, so for longer trips spend a bit more time ensuring your gear and supplies are as light as possible.
Walking times vary greatly depending on your fitness, health, weather, motivation and other things going on around you. The walking time estimates do not include rest time and assume a reasonable level of fitness and mostly clear conditions.
Always allow for extra time in case of extreme weather, injury, illness or other delays. Always be prepared with your bookings and supplies to spend an extra day on track. It is rare for people (especially in the warmer months) to be forced to extend their trip, but be prepared.
Even during the fee-based booking season, you can move at whatever pace you wish, you can stay a few days at one campsite or do a double hut (skip a hut) day. During fee-based booking season, you do need to aim to get to Waterfall Valley (or beyond) on the day you have booked.

(4 Days) Fast pace
(5 Days) Fairly common
(5-6 Days) Whilst Waterfall Valley is Closed
(6-7 Days) Recommended whilst Waterfall Valley is Closed
(6-7 Days) Most popular Itinerary
(7,9 Days) Shortest days
(7-8 Days) Peak Baggers
(7-8 Days) Including Pine Valley
(9-10 Days) Climb every mountain