Depends what level you are at and what you are doing. Far too variable to put into a simple reply. Rest however is a 10/10 whether that be a day, a backoff week, or a month of deloading after half a year. For a beginner, a simple good nights sleep might be enough and they might be able to train 7 daysa week. With a few lighter days. Or maybe 6 days who knows.
Furthermore it is a very personal thing.
I guarantee you though that rest of some kind is essential.
You need to learn about 2 factor theory … the two factors being, that when you train, you increase your fitness levels AND at the same time you increase your fatigue. BUT fatigue goes away real fast, and fitness lasts a lot longer. So you exercise for 2 weeks and the fitness you develop will last for another 6 weeks say, but the tiredness will go away in perhaps 2 days of rest. And THEN you will be able to see how much stronger you are.
I hope that makes sense
If working out mon, weds, fri … well, you might feel like crap on Sat. And sunday feel OK. And monday next, feel like a tiger. Tues, a bit tired but keen. Weds, like a smaller tiger. Thurs, a bit worn down. Fri, like a large tomcat but eager. Sat, like a kitten that was dragged from the lake. you get my drift. you are cycling the fatigue and managing it and all the while, increasing your fitness levels.
Note some people work hard for months and get apparently weaker and weaker or stay the same, then they deload for a week or so and then bam! strong. all that strength accumulated can be displayed. prob. not the best approach if bodybuilding / size is your aim, though.
In my opinion hardgainers are just people who don’t adapt well within the standard routines … they cannot recover from fatigue quick enough to grow. Feeling strong again from a rest is one thing, but being able togrow as well, that takes a bit of a backoff.
Note that volume training and intensity training have different effects on your recovery from fatigue. You can cycle these cleverly. example:
3 weeks volume
1 week lighter volume
3 weeks high intensity
1 week lower intensity
That is pretty much out of Thibs’ book.
So in summary, rest is 10/10 - without it, you will suck big time.