It's hard to say, but I'd say a week at the least, and a month at the most. As much as I'd like to offer BTC back, it's too dangerous for me. (I'm only 16, and don't have a steady income.) But thank you for offering. I appreciate that.
Edit: I'd actually be willing to offer BTC back, but only if it were a small amount. Maybe 1-3 BTC? If this would work for you, let me know.
So in a month if the Bitcoin value increases from like 15 to 30, I would be getting half the Bitcoins I originally loaned to you?