When you are creating a service, you will enter the time required for this service in minutes. This is the base amount of time that this service will take. When you enter your additional questions, by choosing "numeric response" you are asking a question that will customize the duration of a job.
For example, if this appointment was to be for a car service, you could ask "how many tires are we changing today?" If the additional minutes on this question is set to 15, and the customer answers "2 tires", then 30 minutes will be added to the total time of the job.