How long does it take to get a death certificate if the cause of death is uncertain?

It depends on the local law. Each country/state have its own rules.

A death certificate is usually available within a few days even if the cause is still being determined. Under the cause of death section of the death certificate, officials will usually put "Pending" or "Under Investigation". This allows a funeral to take place without an extended waiting period . When the cause of death is eventually determined, such as through toxicology and or microscopic study of the organs, an amendment to the death certificate is filed showing the cause of death.