I'm on board with CtB's timeline, because I agree that the rebuild started with Rielly. Whether the Leafs were shedding core talent fast enough to draft high enough to begin assembling a core trio of top-5 picks (or whatever it takes to really be a year-in, year-out threat) doesn't mean they haven't been enjoying the benefits of a rebuild (drafting high end talent).
That said, I think the timeline is still largely a matter of choice. That is, if the team doesn't trade Kadri, Gardiner, and JvR in the next year or so, I think there could be enough NHL talent on the roster to shelter the arriving Nylander and Marner such that the team is out of the basement (though not a playoff team) in 2016-17. But if they shed Kadri &c. before next fall, then 2016-17 can be just as brutal as 2015-16 is going to be, and they'll have another top 5 pick, some more prospects/picks that'll need a lot of development, and so an extended rebuild, which lines up with what some folks are anticipating and others are advocating (getting a bit better in 2017-18).
[snip - ed.]
Guess my answer is the future is hard to predict, but status quo says (I think) this team isn't an absolute disaster in 2016-17, and they get better from there.