It’s completely up to you! Consider your lesson plan, your student’s level and fluency, as well your personal vision, experience and drive.
Technically you can set 3 lesson duration types - 30-, 45- or 60- minute lessons.
To disable bookings of a certain type of lessons follow these easy steps:
Go to Profile Menu > Tutor Studio > Private lessons Settings > Scheduled & Instant lessons.
There, you will see 3 switches next to each lesson duration option. Initially, they are all enabled, but you can disable any of those, if you wish.