The answer is quite simple: there is no such fixed level, which seperates acceptable from outrageous offers.

As a trader, you break down the APY to daily rates. Then you ask yourself: can I make XX% of profit per day, when averaging over about one week (since day trading doesn't work out well every day). To put this into perspective: If we're at $130 / BTC, the question is: will I be able to lock into a positive price movement of e.g. 3$ each day on average, including all the difficulties of getting in and getting out? Then taking an 600% APY loan would be ok.


PS: I'm living in Europe