Just as an approximate example of what I've found to hold true for me.
If the rifle/ammo combination is a 1 MOA off the bench (front and rear sandbags), I find my abilities to be roughly:
1-1.2 MOA prone w/bi-pod and using off hand as rear rest.
1.5 MOA sitting with bipod
1.5-2 MOA with improvised rest sitting
2-3 MOA standing with improvised rest or kneeling with elbow on knee.
4-6 MOA off-hand.
As such, when hunting, the position I can take dictates the range at which I'll take the shot.
Off-hand shots tend to only be necessary at pretty close range when the window of opportunity may be very short
Standing and using a tree limb or somesuch usually means there is foliage obscuring a shot from a lower position, which typically is also characteristic of a close, make-it-quick shot.
Sitting is common, and if the distance is much over 100 yards, I've found I usually have time to set the bipod and get a solid position. I feel comfortable with shots out to 400 in this position.
Prone doesn't seem to happen with big game hunting where I hunt. Usually just no way to get an angle on the critter from prone in the rockies. For varmints on the plains, however, this is the norm, and praire dogs really aren't safe at any distance I can see them. Inside 300 is virtually 100% hit probablility unless the wind is raging. 400 drops to about 70%, 500 about 40-50%, 600 about 25%, 700 about 15%, 800 about 10%. My longest kill was 782 yards, and it took 6 shots. This is with a .220 Swift and taylored loads. Of course, at 2" wide or less, praire dogs represent about 1/5th of a big game animal's bullseye.
On big game, if I can build a position as steady as prone, I will take a shot out to an absolute maximum of 600 yards. Beyond that, shooting ability becomes less relevant when factors like wind and angles make calculations for which you will not have the variables necessary for an ethical shot. Unlike a praire rat, who is pretty much a gonner if he gets hit at all with a centerfire, a big game animal can very easily run off and die hours, days or weeks later, sufferering all the while, from a bad shot.