I genuinely can't remember whether I have asked this question before. Please forgive me if I already did.

He works 8 hours a day(,) 5 days a week.

Do I need a comma for this kind of construction?

Thanks a dozen!!
You have thrown yourself into a dilemma that needn't be. No comma should be used there, but it looks a little confusing, so I would do this:

He works 8 hours a day and 5 days a week.
Ah, I see! So simple...