Does anyone know, or where to look, about drop out rates from "first one free" offers.

Thinking of advertising a subscription service, online, where they receive first item (mobile content) free. To maximise take up I didn't want to have any minimum subscription length commitment. They are automatically billed weekly till they unsubscribe,

Ideal would be a percentage figure of those who grab freebie and unsubscribe before the 2nd week kicks in.

Help!

Cheers
Col