How do I disable mobile data access for certain apps at certain intervals?
AFWall+ allows creation of multiple profiles and the profiles can be toggled using Tasker. I'm giving you a basic idea now. 


Enable multi-profile support in AFWall+. It's inside the Preferences.
For nighttime, create a profile and allow or disallow the apps which you want/don't want to connect to Internet or to a particular network interface.

(Click image to enlarge)


Once done, create a task in Tasker with the action Plugin &rarr; AFWall+ &rarr; Configuration &rarr; choose your profile.


Now it is up to you how you want this task to be executed. The best option is to create a time triggered profile and assign the task to it. This technique would work for office hours as well. 


  I was also thinking of a possibility to block access for half an hour after a fetch...


This can be done too, now that you've got the idea about AFWall+ profiles. 

Consider this example: If it is about two profiles to be switched after an interval, I would roughly use these actions in a task:


Select an AFWall+ profile of your choice under Plugins
Task &rarr; Wait and make it 30 minutes
Select another AFWall+ profile to be triggered
Add Wait of another 30 minutes
Task &rarr; Goto &rarr; and choose first action.


You just created an  endless task. Attach it to a time triggered profile or any profile of your choice. 

In order to stop that infinitely looping task, create another task and use the action Task &rarr; Stop &rarr; choose the task to be stopped. You've to execute this task of course.