Normally if it ferments too long it will begin to smell and taste like cheese. There is no set time to ferment your milk, my experience is that at 74 deg F, 12-18 hours is sufficient, should smell like yogurt, leave it too long it will go to the cheese smell. IF temp is 65 deg F, more like 48 hours til its ready and smells like yogurt.
They shouldn't go dormant unless in the refrigerator for a couple weeks. If you are starting with grains that you've bought via mail or something, a couple "cover the grains" batches should get them to proper state.