For me, I just have a routine. I work Monday to Friday, so I train those days and take the weekend totally off. Long weekends build in an extra day off once a month or so and vacations add in a couple deloads throughout the year.
There have been times, though, I’ve gone in and felt like crap and I’ll try to get through the main lift, then leave, and there have definitely been other times it just turned into a short mobility/foam rolling session and that was it. Nothing wrong with that, at all. As long as it’s only every couple of months, then I’m cool with it.
I have a routine, so I know if I had a lack of sleep for a couple days (for example), a bad day isn’t unexpected or a cause for concern.
As Voxel said, find a frequency where you feel great and only rarely run down. Maybe that’s 4 days/week, maybe 3, it doesn’t matter in the end. You can’t go off what others do, only what works for you at this time.