You are correct that the null article is more common:
(Google Ngram Viewer)
Both formations are correct, but would be used in different situations.
Whether you're referring to days or hours, you'll either be referring to an indeterminate amount (if you wish to speak of them in general) or a specific set (to speak of fewer than all workdays).
Consider the timespan in each of the following examples:
1. Ben felt a little ill yesterday during the working hours he was assigned to clean-up duty.
2. Ben always feels a little ill during the working hours he's assigned to clean-up duty.
3. Ben feels ill during working hours.
The first example is specific to a subset of the working hours which occurred yesterday, which are themselves a subset of all working hours. The second refers to a subset of working hours, namely all those during which Ben is assigned clean-up duty. The third refers to all working hours.
If you're referring to working hours in general, then the null article is correct:
This door is to remain unlocked during working hours.
If you're constructing a more complex phrase where you want to talk about a specific subset of working hours (or days), then you'll want to use the definite article:
During the working days of summer, there is more demand for ice cream at noon.