There is no set time. Rather it depends on any number of factors, not the least of which is turbulence and the approach. Cutting off new service at T-30 so that everything can be collected no later than T-20 makes sense. At T-40, could have been that a bit of anticipated turbulence or vectoring might have meant clearing the cabin a bit earlier.

Doubt this has to do with fiscal impact as flight crew don't get a bonus for saving on the booze.