I have decreasing hcg levels & no heart beat at 7 wks ultrasound & baby only measured 6 wks. how long does it take for my body to naturally miscarry or will i need a dialation and curettage?

Can take a: couple of weeks, on it's own. Medications such as misoprostil, can help speed the process, a D&C would be faster. The need for a D&C still could rise if bleeding becomes heavy and longer duration.

It’s unpredictable : Assuming you feel well, have no fever, and do not develop extremely heavy bleeding, it is likely that you can avoid a D&C. While generally it may take a week or two before you pass everything, it could happen much sooner than that or much later. I suggest you decide what the longest period is that you are willing to wait and schedule a D&C then. It is easier to have a definite end point.