When setting up bookings with custom time units, the following limits apply:
By Minutes
Minimum duration: 15 minutes
Increment: 5-minute intervals
Maximum duration: 150 minutes
By Hours
Minimum duration: 1 hour
Maximum duration: 23 hours
By Days
Minimum duration: 1 day
Maximum duration: 28 days
Also, for booking by minutes & hours, overnight booking is not allowed. For example, a booking from 11 AM -1 AM next day will not be possible.
You can find more information on custom units and how services are listed on your marketplaces here.
Otherwise, if you are looking to implement other custom units of time, you can find more information on adding your own custom codes here.