Once simmering reduce heat and slow simmer 4-24 hours (some suggest as short as 90 minutes. Uncovered if you have more water than you need (watch it and add boiling water as needed to keep the goodies submerged), or covered if you prefer. Some think > 4 hours is a bit fanatical, they could be right. The point is to extract all of the available collagen and flavor, but there is a point of diminishing returns. If leaving unattended overnight, especially over a gas range, for safety you should use the oven-finish (details below).