For controller info, I would estimate that most autoflight systems would need about 20 seconds from you receiving acknowledgement to seeing a descent on SSR. There's a fair bit of 'knob twiddling' (yes, I know...), confirmation between crews and then a 'softish' autothrottle response.
We've had the discussion before about 'Descend Now' and I much prefer the use of the phrase when required, just to save R/T, especially with so many 'unknown' ATC early descent requirement gates (also a previous thread).