As long as you keep treating it and apply diaper rash cream at every diaper change and make sure you wipe your baby clean even when they just have a pee diaper then the diaper rash even severe diaper rash should clear up in 3 days.
But it can last longer. If the severe diaper rash lasts longer than a week then you might need to take your baby or toddler to the pediatrician so they can prescribe some stronger diaper rash ointment.