DICKUS,
It depends, if I sounded like I was panic mode, you better slow down as fast as you can, If I sound all calm and under control, take your time.
In all seriousness, I use speed control to maintain seperation, not to create seperation. If I ask you to say slow from 250 kts. down to 190 kts, then I'm looking for your speed to be at 190 kts within 6 miles or less, I try to allow one mile per 10 knots of speed reduction.
I also don't expect you to be able to descend and reduce so I will do one or the other first, example, Reduce speed to 210, then descend and maintain, or descend and maintain, and when able reduce speed to.
Since I'm not an en-route person I'm not familiar with mach speeds and how long it takes you guys to slow down.
Mike