Depends if Justin can consolidate the left while keeping the centre. What Hagrid is describing is Grit Inc., when there was no opposition to speak of inside or outside Parliament till 1945, and a mostly useless one thereafter. (Put another way, do you think he's a political genius like King?) He'll definitely win in 2019, beyond that depends on the economy and remaining scandal-free.
His reign as Pm will probably last as long as he has CBC and other major news networks on his jock, his image is doing more for the liberals than any policy or program ever could, if he loses that he could be done very quickly