about mechanism of rainfall
Answers
The basic thing you would need to understand is this thing called the Water Cycle. Water goes from liquid form on the ground (rivers, lakes, seas) into gaseous form (evaporation) or liquid within living organisms (plants, trees - transpiration followed by evaporation) (animals - perspiration followed by evaporation). As the water vapor travels higher up into our atmosphere, it gets cooled down significantly enough to form liquid again. Yes, it is important to note that Clouds that you see is water in liquid form just very tiny droplets that are capable of being suspended by air (so in essence at the cloud level we observe condensation just not enough to make them fall down back to Earth). Next what happens is things gets mixed up up there - water droplets may crash into each other to form bigger droplets but this collection into bigger and bigger droplets is aided by the presence of dust particles (which is why there is a process called Cloud Seeding when people try to make a cloud rain down its content). As the droplets gets big enough, the air finds it harder and harder to keep them up there, and gravity gives it a nice thug to send it dripping downwards. (often this coincides with lots more dust that's why most rain clouds are very dark in color - but yes, you can get rain from normal white clouds too just not too common and generally will be drizzles rather than downpours). Once the water comes back down - it gets fed into the mountains (and sometimes become trapped as solid ice), ground (water table), rivers etc. So the cycle becomes complete (of course a small amount is taken up by living things).
As to how long any rain would last is dependent on many conditions - temperature, wind, amount of total water available within the clouds, pressure etc.. Please take note that is very unlikely that the rain water you get is from anywhere near where you live - a large portion is likely to come from the oceans (large surface area being baked by the Sun throughout the day) and only specific conditions bring all the suspended water particles together near where it would be raining. Overall, there is always a certain amount of water trapped in our atmosphere. So maybe you might want to re-phrase your question - how is the water distributed across the globe (which at the moment is quite hard to estimate but from what we can see there may be patterns related to seasonal fluctuations.)