I believe it will be specified in your company ops manual.
...and should be defined as something like onchocks+30 to ETD-30, halved and added onto the duty day, with a min of 3, max 10 hours.
for example:
Land 1200, depart 1730 = 5h30 on ground
Subtract 30mins from each end of ground time = 4h30
Halve it = 2h15 allowable extension of duty day