Marina,
If you want to see some examples of color studies, I've posted two examples on this forum. Here are the links:
http://forum.portraitartist.com/showthread.php?t=6111
http://forum.portraitartist.com/showthread.php?t=6260
They usually take me about 40-45 minutes, but if you are less impatient and more meticulous than I, then you might want to take a couple of hours.The exact time doesn't matter.