Hi, How Long Should I Expect A Slow-release 20mg Dosage

Hi, how long should I expect a slow-release 20mg dosage of OxyContin to really work as it should? Does it take days, weeks, or should it be immediate? I take it for severe back pain, and it felt as though I got no response from it for a week or more, but now I think I'm getting relief from it, but I'm not certain, because I take a dose of 10-325 Oxycodone in-between as needed up to 3 x day.