If someone is convicted to 10 years on a class C felony, has a scheduled release date in 2020, what is approximately the very earliest they could expect to be out?

The typical guideline to follow is that most sentences come with built-in "good-time credits".  This is normally 15% of the total. Assuming this is available to you, 85% of 120 months is 102 months. Then there is halfway house (if available) which might carve another 3-9 months off the sentence. Be mindful that the halfway house is still a form of incarceration but reserved for inmates with no conduct problems or incident reports.