The minimum I purchase is 1/2 yard. I have pruchased whole bolts if everything is perfect, great fabric ...great price. When I purchase the bolt its becasue I will use it predominetly in the top and use it for backing as well.
As a general personal rule of thumb ...If its a large scale print , I rarely ever purchase under 1 1/2 yards., typically 3 so I can use it in the borders and cut lenghtwise.