The amount of time a student with an extended time accommodation should be provided must be calculated in comparison to the amount of time provided to students without disability accommodations.
For instance, if students without extended time accommodations are given 1 hour to complete an exam, a student in the class with a 150% extended time accommodation should be given 1 hour and 30 minutes.
If you create an exam that you think will take students an hour to complete, and you give the full class three hours to take the exam, the student with a 150% extended time accommodations will be entitled to 4 hours and 30 minutes to take the exam.
The calculation of extended time is not based on how long you think the exam should take students, but rather is based on extending the amount of time provided to students who do not have extended time accommodations.