Depends on the type of pasture in this area. I have one pasture that is 1000 acres. It is native pasture ( never been broke... ever). In an average year we put about 35 pairs on it.( 28 -30 acres per pair) The prairie wool grass is amazing stuff. it grows about 6 inches tall on a poor year and about 10-12 inches in a good year. It matures early and is brown by late July. That is when it really gets good. Cows and calves will come off it butter ball fat. We always try to never over graze native grass as once it is clipped short, it can take up to 5 years to come back to it's original capacity.
Our tame grass/alfalfa pastures will carry a cow/ calf pair on approximately 6 -7 acres... over double that on rotational grazing pastures.
My grandfather used to say that every day you keep a cow off the pasture in the spring will mean two days of extra grazing in the fall. I firmly believe that is true... at least here. I would rather feed my cows an extra 2 or 3 weeks in the spring, that have to start to feed earlier in the fall. Of course a good drought messes everything up!