Asnwer -
infinity
When an object is placed at the principal focus of the concave lens, then image will be formed at the infinity. Because after refraction from the concave lens, light rays become parallel to the axis of the concave lens, hence image will be formed at infinity.
Related Que
When you place an object at the focal point of a concave lens What will be the characteristics of image formed imagines and write?
Answer. When object is at focus of the concave mirror. hence the image is formed at infinity and image is real, inverted and highly magnified.
Que - Is it possible to form a real image using a real object with a concave lens?
Ans - A concave lens cannot form a real image because rays passing through a concave lens bend outward (diverge out) and never meet.