Quite a while. They've been superb and as long as they don't lose anyone to injury they'll be fine.
Thing is, the difference between a top-level side and a side which performs above expectations to match them is the squad depth. They might be having the results and showing the quality right now to keep pace with the top of the league, but this has happened a number of times before and it never quite seems to be able to sustain itself.
By Christmas time and the couple of months which come after, the constant games reach a point where the best players in the upcoming club start to either tail off through tiredness or become more susceptible to injuries, and when that happens they don't have the type of ability in their second string to keep up the results. Eventually they start struggling for goals, or can no longer boss possession in the middle of the pitch, or just can't seem to keep a clean sheet, and one way or another it slowly unravels.
It's like that time when Newcastle were in the top four for pretty much the entire season, about 2-3 years ago. It was going fine for them for ages - longer than most teams get away with, to be honest, but just as the very last few months rolled by then their form slipped and they ended up losing three of their last four including a 4-0 loss to Wigan, plus they went through a rocky patch in February where they dropped points at home to Sunderland and Wolves. They ended up several points short of fourth, plus 20 GD behind the CL teams.
I'd draw a parallel with Southampton last year, though - that will likely be closer to how it ends up, as Newcastle that time did better than most clubs would. Southampton by the middle of January were in 3rd, two points ahead of United and five behind City. They had a five point cushion over the Europa League places. Cue a mid-season crumble, and from their last 16 games they took just 18 points, finishing in 7th at 27 points behind Chelsea, 19 behind City and 10 behind United. I think Leicester will do likewise this year.