When using the crockpot I prefer to always cook everything on high heat so that I know the foods are getting heated to the proper temperature, for food safety purposes.
I see many recipes that call for a low setting of 8-9 hours. Generally, as a rule of thumb low for 8-9 hours = high for 5-6 hours. You can cook it high for at least 5 hours and then turn it down to low if you are home. Some newer crockpots even let you program when you want the temperature to go down. Sometimes I just leave it on high for even 7 hours and the food is still great.