How long does it take for morphine tolerance to decrease?

For several months I was receiving 10mg morphine injections roughly once a week for severe migraine headaches. It was doctor controlled but naturally my body still developed a tolerance to the medication and it was not working very well for me anymore. Approximately how long until my tolerance would decrease so that the same dose may continue to work. I have not had a injection of morphine or narcotic of any kind for about six to seven weeks. Any insight would be helpful but please stick to the question I have asked. Thanks