You always seize the opportunity the moment it present itself.
Once per turn:

• When you roll attack with advantage and both dice hit (met or surpass target's AC) That attack becomes Critical attack.
• If one of the roll is Critical but the other doesn't hit, treat it as normal Critical.
• If one of the roll is Critical and the other also hit, you deal Full damage of critical hit without rolling for damage.
• If you roll 2 critical. you deal Full damage of critical x2 without rolling for damage.

You cannot perform this feat again to the same enemy for the next 24 hours.