How long does it take to get physically add. To pain meds & would it help to switch from opiates to muscle relaxers to relieve chronic lower back pain?

Physical Dependence. Is not addiction and is a normal consequence of chronic opiate use. Addiction is marked by compulsive and uncontrolled use with preoccupation and adverse consequences. A withdrawal syndrome with discontinuation does not imply addiction. Discuss your concerns with your doctor. Non-opiates like muscle relaxants and snri's can help reduce your reliance on opiates to a limited extent.
Not long! Only couple of week is sufficient for some pain relieving pills such as Codeine but you should change your medications under supervision , muscle relaxants do not take the place of opiate analgesics.