How long does it take for an igneous rock to become a metamorphic rock

I believe that is a trick question, "The degree of metamorphism is a function of pressure, heat and time. During orogenic events, the metamorphism of the rocks can take 100's of thousand to millions of years. Too much heat cannot be applied at once since the rock will melt and recrystallize into an igneous rock" i've found those same answers on other websites as well. meaning there really isn't a right answer.

The degree of metamorphism is a function of pressure, heat and time. During orogenic events, the metamorphism of the rocks can take 100's of thousand to millions of years. Too much heat cannot be applied at once since the rock will melt and recrystallize into an igneous rock.