Helen was not an historical person: she is a part of the Trojan Cycle, a sprawling collection of early Greek myths pertaining to the mythical Trojan War. We can very roughly and hesitantly posit that there was possibly some kind of actual conflict centered on the city of Troy, and that the conflict probably happened at the end of the historical Bronze Age. The actual site did undergo a series of destructions beginning in the 12th and 11th centuries BCE, which is when (roughly) the Bronze Age is coming to an end in the Aegean. Some internal items of evidence in the Homeric epics point to a date around this time (for instance, the helmet made of boars' tusks). We might even posit that there is some sort of "cultural memory" of an actual kidnapping of a Greek aristocratic woman which was in some way related to the theoretical conflict. But that is as far as any dating can go.