Depends on cloud height, density, humidity, wind speed, and I'm sure many other things. If you knew that there was no wind, not an abnormal amount of humidity, and time it fell from a cloud of a known height you could calculate it's terminal velocity, which it would reach in a few seconds, then use that speed to calculate how long it would take to travel to the ground.
Chat with our AI personalities
Normally it would take 1 hour, but a flood, could take a few days.
If it carries on raining, bad luck, but if its sunny, it shouldn't take long, the time would fly by.