How To Understand When To Start The Pain Relief Medications?

i am an Enrolled nursing student and am having difficulty understanding the use of pain relief medications used after surgery. I am doing placement on a surgical ward that deals with hips, knees and shoulder replacement/repair and arthroscopes etc. Firstly it seems they start with paracetamol and then work up to the more opioid medications to help with pain..because i don t know a thing about anaesthesia, how will I understand when to start the pain relief medications after the patient is returned to the ward?? like the time frame of how the anaesthesia wear off??