Who maintains your computers/networks/wireless/servers/WordPress etc, etc, etc? If you don’t have someone available when a problem arises then you are at the mercy of who has an open schedule.
How much time should you allocate?
This all depends on what type of technology you have in your office.
Let’s pretend you have 1 Windows server and 10 network computers. You should budget at least 2 hours per month just for monitoring the server and the computers health. This though is just a bare minimum just for monitoring your networks health and does not allow for tech support questions and extra availability.
What are you really paying for with service contracts?
You are paying for maintenance AND AVAILABILITY