Pretty much everything in our solar system has collisions with smaller objects that lead to an increase in the mass of the larger. With Pluto being one of the largest things in the area of which it resides, and being surrounded by small stuff. Assuming that it does not hit Neptune, which seems unlikely

How long will it take Pluto to grow to planet size?

Clarify or what ever the name of the largest object in the Kuiper belt turns out to be, at the time it reaches the IAU definition of planet

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