Using Customer service schedule in custom entities

Customer service schedule perfectly works with SLA and cases.

But I want to use it on opportunity or some custom entity.

There I have the calculated data and time field and need to calculate time for some event, using working hours from Customer service schedule.

Something like this :   Expected time in this field = AddHours(5 working hours using information from Customer service schedule, custom field).

For example, now is Friday 16:00 . Working hours in Customer service schedule are from 9:00 till 17:00 from Monday till Friday. If process needs 5 working hours, the end of the process will be on Monday 13:00.

Please, help, how to make that work?

Thanks!!!