We do not know for certain because we have never seen this process .We think that as the sun began to form out of its primordial, spinning disk, that the material in other parts of the disk began to grow in size from small meteors to asteroids and then to large bodies hundreds of kilometers across. These bodies then collided to form proto-planets whose strong gravity fields made them very effective vacuum cleaners in their orbits causing them to grow 'explosively' in mass in only a few 100,000 years. Evidently, to start the process going, you need to make certain that the disk is not too turbulent because this causes the small particles to hit each other too fast and shatters them before they can grow big enough.
Planets may only need less than 1 million years to grow to their present sizes. Jupiter's core is so hot that we think it only took 100,000 years to form this planet starting from dust and gas.