for this you have to inherit the fullcalendar.js file which is in web_calendar module.

for the reference please refer this file addons-->web_calendar-->static-->lib-->fullcalendar-->js-->fullcalendar.js

and do the required changes in following code:

setDefaults({

allDaySlot: true,

allDayText: 'all-day',

firstHour: 6,

slotMinutes: 30,

defaultEventMinutes: 120,

axisFormat: 'h(:mm)tt',

timeFormat: {

agenda: 'h:mm{ - h:mm}'

},

dragOpacity: {

agenda: .5

},

minTime: 0,

maxTime: 24,

slotEventOverlap: true

});

do change in mintime and maxtime as per your requirement.

hope this will help