Not sure there is a best design for the sessions, more laps just gives the cars a bigger chance of doing damage to themselves. However an extra 5 min track time also gives a lot more hot laps.
eg. With my perferred 3 lap warm up each session, assuming a 1.2min lap gives 5-6 hot laps in 10 mins, where as an extra 5 min gives 9-10 hot laps.
4 sessions 15 min + 5min cool down-swap over, would give a greater opportunity to drive into town for extra fuel and a bigger break for lunch. Overall currently we get 35-42 hot laps a day, 15 min sessions we would get 42-52 (assuming 3 lap warm ups each session and 1.2 min laps).
I am happy as it is, but understand others would want to try longer sessions.
On another item, any thought of sharing the load at the day, eg splitting the pit wall duties into two one morning one afternoon?