It depends on the area, what type of grass the pasture has, the soil fertility, and a lot of other factors.
If she's in a drought type area with a lot of weeds or un-palatable grasses that may not be enough. Like where I live you can support one horse for every 1-2 acres (with proper fertilization of the soil & pasture care) we can up that by rotational grazing, but you want to be careful then that you don't over stress the pasture. Is the pasture mostly grass or is it one of those "wooded pastures" basically in a forest? If it is mostly woods the actual usable grass is much less than 10 acres. Peggysue is correct about a lot of pastures not being nutritionally enough now but that is due to our (horse peoples) tendency to WAY overstock pastures (I know lots of people that have 2-3x as many horses as they should on tiny, mud filled pastures)... and the fact that very few of us soil test and fertilize properly.
I don't know a lot about CA pasture loads though so you guys could be totally different in terms of pasture loads.
One extra thing too for the health of your pasture (and therefore your horses) you should take them off of it once it is below 3". The best height to turn out on is 6-8" of grass (dependent upon the grass species) and to take them off when it hits 3-4". Basically you want to put them on before it seeds/flowers and take them off before you hurt the grasses ability to maintain itself. This way you reduce stress on the grass, because stressed grass = room for weeds to move in = less grass = horses eating things they shouldn't.
I should show my Equine Nutrition prof this, maybe extra points to prove I learned lots in class?