2.5 miles
In the absence of any ATC speed control and any formal speed instruction printed on the arrival chart, I fly the speed I feel appropriate on the day. This may or may not fit in with other traffic so you guys need to tell us. (To digress, all too often high performance turboprops are shoved to the back of the queue and given a restrictive speed over 20nm out, when they are capable of maintaining 250kts until 10-12nm. Are all ATCOs aware just how early a 737 / A319 etc. needs to reduce to 210kts and below in comparison, not least the additional vortex gap they leave behind?)
My point is that, when I am so often given a speed of 210kts on arrival at the zone and receive no speed instruction thereafter, we cannot possibly maintain this until 4 DME. I see your point clearly but the fact remains many of you guys issue a speed when we are far out to suit your flow, and never get around to reducing us for the approach. It's around 8 DME we will reduce to circa 180kts if we haven't heard anything further from you. I certainly don't condone the ignorance by pilots of speed instructions when they go something along the lines of 250 / 210 / 180 / 160 to 4 DME. That's plain cheating and I share your annoyance.
To give you my take on your last question, although I have never seen official guidance, in practice if a slower speed is issued by ATC then I would configure the aircraft to start reducing immediately, with a view to being locked on the speed within 30secs, although this may take up to a minute on a medium jet.