I've been wondering this and been searching this site and I think I didn't find any answers. I am asking if each turn if one attempts to stay hidden, must he use action every round (out of combat or in combat) in order to stay hidden?
This seems dumb as non-rogues cannot ever gain a stealth advantage on an attack roll for being unnoticed, due to not being able to both stealth and attack in the same round.
No — you only need to reroll if you do something that the DM determines would expose your location, like make a loud noise or step into clear visibility with no distractions. In that case, if the circumstances allow, you can use your action to hide again. The rules intentionally leave the exact determination up to the DM, but the basic idea is that your one roll covers staying quiet and out of sight over a period of time.
This podcast on the official D&D site has Jeremy Crawford giving about 10 minutes of explanation of stealth. It starts at about 10 minutes in. There is some interesting background detail about the rules in this edition compared to previous edition, and the deliberate design to make the stealth system more up to the DM and less "mechanistic".
The first five minutes really focuses on explaining that in this edition, stealth is by design heavily dependent on the DM's judgment. Then there's a nice explanation of stealth vs perception — and specifically that it's normally compared to passive perception.
But it also addresses this particular question specifically, at about 17 minutes in.
Crawford: As soon as the player makes that check, that Dexterity (Stealth) check, they keep that result, whatever the total is, from that check, they keep it until someone discovers them, or they decide to stop hiding. So, this is relevant particularly in combat, because one of the actions you can take in combat on your turn is to hide. You need to spend your whole action doing it. And this rule means, if let's say you want to hide multiple rounds, you don't keep making Dexterity (Stealth) checks round after round — you just make it once. Basically what this means is you only have to spend one action trying to hide, and then once you've done it, you keep whatever that result was, until you're no longer hidden, and again that's either because you've run out of hiding, or you made a loud noise, or someone discovered you. As soon as that happens, even one person discovers you, basically that nullifies whatever you rolled, and if you want to hide again you're going to have to make another check. And what that means in combat, that means you're going to have to spend your action doing that again.
Interviewer: What if for example, though, you hide around a corner of a hallway from somebody, from an enemy that's in the hallway, you get that, so you're hiding around a corner. You're still hidden, but you decide to move, and go into a room as well. Do you still use the initial stealth check, or do you have to roll again when you're moving?
Crawford: Uh, you do not have to roll again.
Interviewer: Interesting!
Crawford: Yeah. The main thing you have to do, once you have made your check, the main thing you have to do to remain hidden is make sure people can't see you clearly, and make sure you're not making a bunch of noise. And, the number that you get from your check really determines how well are you succeeding at those things, how well are you succeeding at staying out of site and staying quiet. And that's ultimately what "hiding" means. [...]
Interviewer: Now, is that rule to speed up play, as far as not making you sure you have to roll a stealth check every round?
Crawford: Yeah, and we in general don't want people to have to make a bunch of rolls for really what is a single process. It's only when basically that process ends, is it time for a new roll.
Interviewer: Got it.
Crawford: And, also, we want you to be able to do other things on your turn, especially when we switch to the combat context. We don't want to make hiding so onerous that it essentially becomes something you never want to do.
Interviewer: Right, that's the only thing you can do on your round.
Crawford: Right, right.
Interviewer: It quickly becomes un-fun.
Crawford: And, there are a number of things that have to stay true for you to remain hidden, once you've made your check. And that again is you have to make sure you're not sort of just standing right out in the open with no visual obstruction at all, and you're not, like, screaming or, you know, shattering things and whatnot.
There's more, but that's the key part relevant here.
Generally no, you do not make a check every turn.
When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence. (PHB 177, emphasis mine)
Getting discovered is pretty straightforward. (Do not forget that making an attack will reveal your position.) It is not so clear what "stop hiding" means. Obviously you can give up stealth voluntarily. But I would say that you also stop hiding if ever during your movement you are in a position where you could not attempt hiding. Like moving 15+ft from cover to cover. Depending on the circumstances you would either get discovered or need to roll a new check. Keep in mind that this last part is my ruling, so your DM might decide otherwise.
