If you want to set 8 -10 as working hours, File, Options, Calendar options - set the start and end time.
If you want to see 8 10 in the calendar's time scale - this is controled by how much space there is. Bigger time scale (30 min instead of 15), and bigger window will change how many hours show. It goes start of work day then shows as many hours as possible and if all working hours are visible, it adds some in the morning and some at night. To see 8A - 10P, you'll probably want to set 10 pm as the end of the work day or use the hour scale.