Have you been physically addicted to opiates before?
Generally, it takes a few weeks of DAILY use of opiates to develop a physical dependence on them, at least the first time one develops said dependence. After the first time, however, physical dependence tends to build much more quickly, since your brain, in effect, already has learned how to become dependent. So, the second or third time around, you can become at least slightly physically dependent after just several days of consecutive use. It takes different amounts of time for different people, however.
You took a day off this week, though, and that should be enough to avoid becoming physically dependent. If you really want to be safe, limit your use to every other day or, better yet, every few days. The problem is that its hard to do this, especially when you have the drugs right there with you.