Cover and leave at room temperature for 8-24 hours. The exact time will depend on the culture you use and how sour you want your whipping cream. Don’t over culture it as it will curd.
Culturing whipping cream can result in a thick sour cream consistency. However, I usually avoid culturing cream that long, so that I still have a pouring style cream that can be whipped up for desserts or stirred into a soup, etc.
See section below for details on each of the different mesophilic dairy cultures. They will each bring their own particular flavour to the whipping cream.