Hello Cem-Kaan,

Thank you for purchasing Amelia, and for reaching out to us.

You can do all of that in two different ways:

For example, you can set up the time slot for one hour, and you can add extras to that services for example: if your appointment is by default one hour but the client wants an appointment of two or three hours he can add one or two extras (one or two appointments for one hour) to the default appointment. By selecting these extras, your Customers will be able to change the Service duration to 2-3-4 hours.

And the second option packages of services.

Packages of Services is the only feature that is not included in the Basic license, the rest of the features are absolutely identical. Packages of Services is a feature that allows creating packages of multiple appointments (of the same or different services). 

In that case, the services that are included in the package and the number of appointments will be fixed, and the price will be set for the whole package. Customers will be able to book included Appointments on any day, with no strict recurrence. You can see more information here.