How long does it take for extended release morphine to start working?

Usually 20-30 minutes. In a trauma situation, paramedics will usually give you a drug called Fentanyl, which is 100x stronger than morphine and starts to take effect usually 30 seconds to a minute in. Afterwards, they will also give you Morphine. Morphine takes longer to kick in but lasts longer than Fentanyl so once the Fentanyl wears off, you still have a good dose of pain killers in you via Morphine.