What should I do if the customer only has to choose the start time, but the duration of the service is fixed?

You can specify a fixed rental duration in the resource settings. But changing the duration depending on selected price options and fields is not supported. Initially, the customer selects the start and end time. And only after that he can choose price options and additional fields.
So let's create a rental with a fixed duration. Create a resource — Rental. In the rule of work, specify the working time schedule and necessary rental steps.
Now open the parameters of the resource. Set the Fixed duration checkbox and specify the number of days, hours, or minutes.
See the demo for a resource having a fixed duration of 3 hours:
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