i also thought i had seen it said 24 hrs...
i got this info from the developer's terms of use at this address:
http://www.android.com/us/developer-distribution-agreement.html3.4 Special Refund Requirements. The Payment Processor’s standard terms and conditions regarding refunds will apply except the following terms apply to your distribution of Products on the Market.
Products that can be previewed by the buyer (such as ringtones and wallpapers): No refund is required or allowed.
Products that cannot be previewed by the buyer (such as applications): You authorize Google to give the buyer a full refund of the Product price if the buyer requests the refund within 48 hours after purchase.Except in cases when multiple disputes are initiated by a user with abnormal dispute history, billing disputes received by Payment Processor for Products sold for less than $10 may be automatically charged back to the Developer, in addition to any handling fees charged by the Payment Processor. so what i'm understanding on this is that they might be keeping track of people who do exactly what you said .....pay for apps, back them up, and then ask for a refund....
idk I might be wrong....
