Whatever days you see fit.
I’ve found that they’re best used as necessary. I used to draw two baths (4 bedroom, two bath house), back when I lived with my mom, and I’d hop back and forth between scalding hot and freezing cold baths for as long as I wanted. Sometimes it would be 30 mins, sometimes a full hour or a bit more. 2-3 days a week is a decent frequency for me, sometimes I don’t need them for weeks at a time, sometimes I’ll do them for a few days back to back within a week.
Heat does a good job of soothing my lower back, and the cold baths numb out my cranky knees as well. But above all, the baths knock me out and I manage some really good nights of sleep that aid in recovery. When I wake up all the aches and pains am either go away or are hardly noticeable. If you can use the baths as a means of aiding your sleep, that would be good. First thing in the morning might be beneficial too, but you’d have to try it out and see.
So it’s really up to you how you go about it.