By default, every Bonsai account will have a single event type they can use for Scheduling client meetings/calls/etc. Currently, the default meeting time is 30 minutes.
All you need to do to change this duration is click the dropdown menu and select "Edit Event Type". Here you will see the modal to edit your event type that clients can book. You can edit the duration of the event (to 90 minutes, for example) as well as a variety of other details.
Alternatively, you can always create a new event type for 45 minutes, 90 minutes, etc and use the different booking pages separately.