Keyhole lips get filed under "fuller lips", but the part people actually want is the small opening in the middle when the mouth is relaxed. Lose that and it's just a bigger lip.
That's why placement matters more than amount here. The IV Lounge describes adding fullness at the centre while keeping the opening. InjectCo describes that plus placement toward the outer lips. Two plans, one name.
The clinic guides also say too much fullness can close the gap you were after. So the question worth asking out loud is: how would you keep the opening on my lips?
Some follow-ups, if the first answer is vague:
Reported wear is 6 to 12 months. There's no keyhole-specific amount in the sources, so any number you see online is someone's guess.
If you want a picture to talk through, here's how LookPrep draws keyhole lips on demo faces. A gap in a drawing shows the shape. It can't tell you whether treatment would create or keep one on you.
Sofia R., LookPrep