Well, I only flew medium jets rather than 'heavy metal' but I can give you an outline.
Initially, a descent will be conducted at a fixed mach number. As the aircraft descends into gradually warmer air this equates to an increase in TAS so the aircraft needs to accelerate and thus the rate of descent will be relatively high.
This is all highly variable depending on type and load, but an typical ROD might be around 3000 fpm with a typical load.
At some point IAS becomes more limiting than Mach number and the profile will change as a result. Typically an automatic change over from holding Mach to holding IAS occurs. Now you are looking at a situation where a fixed IAS, since you are descending into denser air, must equate to a decrease in TAS, i.e. the aircraft is now decelerating. As a result, the ROD will decrease.
The differences between these two stages can be quite marked - perhaps 1,000 fpm difference.
Another factor that may significantly affect the descent rate will be Icing conditions. To supply enough bleed air for the ice protection systems it may be necessary for the flight idle rpm to be raised (either automatically or by flight crew input). Hence residual thrust is increased and Rate of Descend decreases as a result.
Once you get under ATC speed control, reduction in IAS will result in very modest descent rates - perhaps a few hundred feet per minute. As to when this will happen - depends hugely on the airspace.
Is that any use?
pb