How bad does a sunburn have to be to have melanoma?

There really is no answer to that question. In fact, the relationship between sunlight (over)exposure and melanoma development is still being figured out. Many melanomas develop in areas that are shielded from the sun most of the time, and in fact, some occur in areas that receive no sunlight at all. This is not the case for the more common skin cancers, such as basal cell cancer or squamous cell cancer, in which the total amount of sunlight that is obtained is correlated with the development of the cancer. Having said all of that, there is a correlation between bad sunburns when younger and the later development of melanoma; but as to the amount of sunlight required to cause the melanoma, the answer is still unknown.