If there was a real reason for asking this, such as writing a sci-fi story, then taking "Mini time" to be the same as "Normal time" is usually acceptable. However, if you wanted to be totally realistic, if you know what I mean, then there aren't even enough atoms in a spec of dust to create a miniature galaxy, planet or even a miniature ant. These things are already as small as they can possibly be in order to behave as they do. Spec-of-dust-sized humans would die if they had the same dimensions as us because their cells (being of normal size) would suddenly be huge in comparison with their bodies, and it would just be too weird. If you're designing a microscopic world, you can almost rule out intelligent life, unless a machine is invented that is more compact and efficient than an actual brain'so, getting back to the question, why not time how long it takes for an amoeba to rotate its flagella 360� and use that?