I teach 3 classes during the week that start at 5:45 a.m. They are my biggest classes because many people want to get their workout done for the day and then go to work. So , I guess to answer your question, probably somewhere around 5:30 or 6 a.m. would be my guess for the earliest start. Just depends on your location and market.
Your time of 10 will also work well for those with a flexible schedule and stay at home moms.
Hope this helps and good luck!