The question assumes the answer is a number of days, which is where most rest day advice goes wrong. Your body does not recover on a weekly schedule. It recovers on several overlapping clocks, and a fixed rule ignores all of them.
Three clocks, none of them weekly
Muscle tissue. A muscle group trained hard needs roughly 48 to 72 hours before it can be loaded hard again productively. That is per group, not per body. Your quads being unavailable says nothing about your back.
The nervous system. Heavy loads near maximum and very high intensity intervals take a systemic toll that recovers on its own timeline, often longer than the muscle itself.
Accumulated load. The slowest clock. Weeks of consistently high volume produce a fatigue debt that no single day clears, and which is what deload weeks exist for.
A rule like “two rest days a week” is an attempt to average over all three. It works acceptably for someone doing moderate mixed training and poorly for everyone else.
What the muscle clock implies
If a group needs 48 to 72 hours, the practical consequence is that you do not need rest days at all so much as you need to rotate what you train. Legs Monday and Thursday, upper body Tuesday and Friday, and both groups get their 72 hours while you train four days.
This is the entire argument for organising a split around recovery rather than around convenience, which is worked through in building a split around muscle recovery.
A default that works, then adjust
For most people combining strength work and cardio, a reasonable starting structure is one complete rest day, one or two genuinely easy days, and three to four hard sessions with the muscle groups rotated so nothing gets loaded twice inside 48 hours.
Then adjust based on evidence rather than on the plan. The things worth watching:
Resting heart rate trend. Drifting several beats above your baseline across a week is the single most useful objective signal.
HRV rebound. A drop after hard work is expected. A failure to come back after easy days is not.
Perceived effort at known paces. When a familiar session feels much harder at the same heart rate, that is real information.
Sleep quality falling while fatigue rises. A characteristic and often early combination, covered in the signs of overtraining.
Who needs more rest than the default
Older athletes, people new to training, anyone in a calorie deficit, anyone sleeping badly, and anyone with substantial stress outside training. Recovery capacity is not a fixed property of your sport, and the two things that most reliably reduce it are under-eating and under-sleeping.
Conversely, well-trained athletes doing mostly easy volume can often train daily indefinitely. The variable that matters is intensity distribution, not day count.
On whether a rest day should involve moving at all, see active recovery vs a full rest day.