A recent answer on this site says that "People have always prayed to Mary as a chief intercessor on their behalf." Such a statement will surely seem excessive to Protestants, who would normally characterize the idea of Mary interceding for the living as a (negative) development that took place centuries after Christ.
I doubt that debate will get resolved here, but it still makes me wonder – what is the earliest known instance of a petition directed to Mary? I am looking for a clear statement where Mary is the actual target of a petition, not that she is tangentially referenced in association with a petition. I'm not looking for statements of praise directed at Mary.
Checking quickly in Everett Ferguson's Church History, I found the following, which may prove useful as a starting point for answers:
Mary was invoked in prayer among Greek speakers in the third or fourth century, but the first Latin hymn involving an address to Mary is from the fifth century. (16.2.C)
Notice however that this quote lacks two things that I'm looking for in answers – it doesn't specify that the prayers were petitions, and it doesn't state the documents in which these prayers were recorded.
We have a similar question, When is the first documented case of Christians praying to the dead saints? However it does not specifically address the question of prayer to Mary, and though Mary is mentioned several times in one answer, she is only praised, never petitioned, in the prayers quoted there (such as from the Liturgy of St. James).