I reckon it will be 3/4 months till it drops to £500
Note 4 has been out for 5 months now and is only really just hitting £450 in the last month or so and I can't see it dropping to £400 for about another month.
IMO the early adopters are being ripped of big time, Samsung and the shops/etailers (more so the latter) know that people will buy their products regardless of cost etc. so they are just simply trying to make a quick cash in from those guys, if the pre-order numbers/interest was crap, it is guaranteed the price would drop massively after all, it only costs about £100-200 to make top end devices iirc.