If it is a cylinder, or a Eurocylinder with a Yale-shaped key as widely used on plastic doors, it takes less time than carrying the toolbox out of the van and writing the bill.
It is widely said that some unskilled operators like to spin it out into a couple of hours to bump up the callout charge.
If it is a mortice deadlock with a chubb-shaped key it takes a little longer because the correct spot has to be measured and marked quite accurately, and the case of a BS lock is armoured to make drilling more difficult.