They refund based on:
A recent purchase before item goes on sale or free offer (e.g., Alistar bought a day before YouTube offer, Kennen bought hours before sale is announced)
Dissatisfied with your purchase, in which a one-time offer of up to three items bought within a 30 day period may be refunded (that is, if you do not refund three items at once, this entire thing is gone)
However, it's Riot, so often you'll get cases where people refund multiple times and get away with it.
But for something like ten+ items, I doubt it.