The rate at which physical dependence develops is contingent on several variables (some of which you've already mentioned), such as the strength of the opioid used, its abuse potential (how rewarding and reinforcing a stimulus the drug is), the dosage used, the manner of use (i.e., as per the doctor's instructions or just to get high), the consistency of use, the frequency of use, the frequency of redosing, the interval of time provided between doses, the user's tolerance, the user's physiology and gender and body size, and so forth.
One cannot solve the question using most of these variables without knowing the particular opioid used, as a given answer is contingent on the given drug, as there is little congruence between different opioids and their rates of developing physical dependence.
But, in the case of <4 mg dilaudid, supposing for some variables like body dimensions and sex, I would presume an interval around one month of regular use with a frequency of ~3 doses per day would be sufficient to establish a physical addiction in the user.
As for <10 mg oxy, I cannot say because I'm not sure if you mean oxycodone or oxymorphone.