It's a while since I've checked but the UK AIP used to say set QNH as soon as cleared to an altitude unless further flight level vacating reports are required.
So for example if at FL360 and ATC say "Descend now 2,500 feet QNH XXX" and nothing else you would immediately set QNH.
The ICAO PAN OPS last time I checked says in this case set QNH unless further level flight above the TL is anticipated.
As has been previously stated airlines have SOPs around this and also have a third altimeter where QNH can be set at top of descent.
Thinking Threat and Error Management one issue amongst others is busting MSA on descent if QNH not set and risk of CFIT when pressure is significantly below 1013.