My record is Knight's Life. The first bonus I ever got on it took just under 4000 spins. Wierdly enough though, it didn't wreck my balance at all. I was down maybe 250x. Basegame sustain on that slot is amazing at times. When I got the bonus it was 20x.
I have probably made profit on the slot over time though. My suggestion with these long-winded bonus slots is to start at 25-50% of your normal stake for a while, until you feel like you can raise it after a heavy amount of spins. With bonanza I plan for 500 spins. Starting low, I raise after 200 spins, then 400, with the last 100 being a bit higher. Hopefully you hit the bonus in a decent stake. You can always mix it up a bit too.