There isn't too many features/bug fixes that I can do in less than one hour (even w/out distractions), so in my case it should be a 60 to 90 minutes session. Also, what I do to get me 'in the zone' is a ritual: close email/social apps. Silence phones. Close my eyes for a few seconds planning what I have to do. Start coding. Maybe you should find a way to implement guided rituals before the session starts? Just a thought.