Depends on Type. It all depends on the type of morphine one is taking. There is an immediate release form (ir) and several extended release (er) forms (ms contin, kaidan, avinza, morphine cr). You can expect most ir formulation to last 4-6 hours. Depending on the patient's metabolism some last a little less or more. The er forms last any where from 12 -24 hours depending on the brand.