there is no such thing as adequate and there is definitely such a thing at the same time. a rifle can hold 6 MOA and be considered adequate for hunting up to 100 yards as the kill zone on a deer is only 6 inches. however if you plan on shooting out to 200 yards you'll have to have a rifle capable of 3 MOA as the target is moved back but does not grow any larger. if you plan on shooting out to 300 yards, then your rifle will have to do 2 MOA. so based on those ranges you'll have to look at the maximum possible range you could be shooting at and plan accordingly.
then there is personal accuracy. no shooter is as solid as a bench rest, you are going to wobble some, no matter how stable your stance is. if your gun shoots 1 MOA groups but you have a 3 MOA wobble, you're still going to have to plan for a possible 4 inch spread at 100 yards. case in point, I shot my deer this year with my new AR15, it shoots 1 1/4 inch groups from a shooting bench at 100 yards(with the ammo I was hunting with). I shot my deer at 138 yards meaning I roughly should have been an even 2 inch group. however even from the very solid sitting position, I hit about 3 inches from where I was aiming because my wobble added to the variance in my guns inherent accuracy. I still hit the deer in the kill zone, but if I had been less careful in my shot because I figured my bullet would hit within 2 inches of where I aimed, I could have wounded it and ended up tracking. the same could be said if I was hunting with my Mosin nagant which normally will hold no better than 4 MOA, added to my 1 inch wobble could have had a possible 7 inch spread and missed the vital organs completely.
now if the shooter gun combo is able to keep their group size with any shooting position under 6 inches at whatever the max range you will be hunting, that is adequate. however you should never strive to be adequate, there is always room for improvement, never stop trying to get your groups smaller. the instant you are ok with being able to hit that paper plate at 100 yards, your marksman skills will decline rapidly and you'll end up maiming a lot of animals which will run off and die somewhere else in agony.
lecture aside... 6 inches at whatever range you'll be shooting is the generally accepted requisite.