Once a round bale is out in your pasture, how soon mold becomes a problem is totally dependent on your weather. The only time we have to watch for mold here is in the hot, humid, summer months after lots of rain (the perfect mold environment) when there is still a lot of pasture available, I.e. They eat less hay and a round bale can last 3 weeks. Mold has never been a problem when the weather is dry or during the winter (where the mares go through a round bale in 6-7 days).
With the weather around here, you generally come out ahead $$-wise using round bales starting at 3 horses....they can eat it fast enough so mold isn't a problem.
If you do use round bales, in addition to a hay ring, if you put the bale on a pallet to keep it off the ground and daily rake the tossed out hay and throw it back in the ring, you'll drastically reduce your waste. Our mares will eat every bit down to the pallets and our waste is < 5%.