I subscribed on 15/3 (with the 1.5GB free offer) and I just did a check as well and got:
"...1536,00MB ΠΟΥ ΙΣΧΥΟΥΝ ΕΩΣ 31/05/2010"
(which I can't use until August when I'll be in Greece
)
Yes it's based per calendar month according to their initial announcement.