A study assessing 6 months (n=112) and 12 months (n=96) of melatonin treatment of 2mg in a controlled release capsule, taken 1-2 hours prior to sleep, people aged 20-80 with primary insomnia failed to show any tolerance to the treatment. The authors noted a slight sensitization to the effects of melatonin at the 3-4 month period, which was attributed to better entrainment of the circadian rhythm. These results have been replicated in another study, lasting 6 months, with a sample of 791 people. No melatonin tolerance due to usage was observed. Another study, lasting 6 months with a sample size of 421 people also replicated these results.
Source: http://examine.com/supplements/Melatonin/#summary15-1