Where I work, the minimum time between appointments is 15 minutes (more sometimes depending on when the clients are available). I greet the client 5 min before the appointment and do my intake, then let them change. Depending on the particular client and the extent of their intake, I'm usually starting no later than 5 minutes into their hour. I end each session at the 60 min mark, on the dot.
On occasion I will go a *little* over, but that depends on the circumstances of the client and treatment, and whether or not I KNOW that someone hasn't been booked for the next slot while I was working (a fair portion of my clients are walk in club members). Each of those 15 min in between appointments is critical.
When I first started, I made a promise to myself that if I was to expect clients to respect my time as their therapist, I was going to respect their time as the client, and never make them wait. If they show up late for an appointment, then they've lost that time and the work will be adjusted as necessary so I can still do quality work with the time I have, instead of doing a rush job.