According to wikipedia, microscopic is considered up to 1 micron. [1] According to the Schwartzchild formula [2] r=(2Gm)/(c^2) so if I take the largest black hole the definition would allow, the mass would be about 10% of the moon. You'd feel 1G at about 60km distance because of it. But indeed, it would fall and take about 20 minutes to reach the core of the planet, while displacing a whooping 1gram of matter. Kind of funny how wierd this whole thing is.

[1] https://en.wikipedia.org/wiki/Microscopic_scale

[2] https://en.wikipedia.org/wiki/Schwarzschild_radius