Unusually for me, I won't be voting in this poll. Watching the debate going back and forth, it strikes me that it's one of those questions which there is no way to answer with anything but a guess, and that in failing to recognize that people are becoming at once increasingly polarized and less rational.
Even if you accept the "slow transition" logic (...which I absolutely do not, BTW) expecting an ice-free north pole not to happen until after 2030 would IMO be like thinking you can play roulette all night at an off-strip casino and not get hit with a zero, or double-zero, or triple-zero. Even given current FYI thicknesses an ice-free north pole wouldn't have been impossible at any time from ~2007 onwards, and in all likelihood it will become increasingly probable year upon year going forward.
At the same time, given what we already know about 2016, I'd say 30-50% for this year and the same for 2017. W.r.t 2018 on I'd guess a 1 in 4 chance, given that it doesn't happen in 2016 or 2017. So IMO it will be sooner, rather than later.