So, recently I came to know that this asteroid is orbiting Earth from past 3 years and still it is orbiting. It is kind of a mini Moon to the Earth know to have come from asteroid-belt.
Is there any threat of collision by 2020-CD3 on Earth and how long it will take to escape the Earth's gravity?
There is no chance of a collision in the short term. 2020 CD3 is in a rather chaotic orbit of the Earth that extends well beyond the moon and doesn't come very close to Earth Each orbit is different, but no orbit brings it closer than the moon. The orbit is irregular because it is perturbed both by the moon and by the gravity of the sun. At its furthest point from the Earth. The gravity field that it moves in is not a simple "inverse square law" because of the combination of Earth, Moon and Sun.
It is likely to escape Earth in the next few months, and it will go back to be a sun orbiting asteroid, we know this because we understand how gravity works, so we can predict its future path. Its future path involves it leaving the neigbourhood of the Earth. However as its orbit does bring it close to the Earth's orbit there is a chance of it colliding with the Earth at some point in the future. It would make a bright fireball, but would not be dangerous, as it is only a couple of metres across. Objects like this collide with the Earth frequently.
Its orbit is not yet well enough known to determine exactly how it will return to a solar orbit, and so we can't yet predict whether it will be temporarily captured or even collide with Earth on its next close approach. But the probability of a collision is low.
