Dear Jamie, Stalactites and stalagmites form in a simple, two step process. First, it rains, and the rain drips through the ground, absorbing carbon from dead leaves and from carbon dioxide in the atmosphere, forming a weak carbonic acid. This solution moves down under the force of gravity.
Stalactites usually form from slow drips.
If the water is dripping fast or running, it hits the ground before it gives up all its carbon dioxide and the newly formed calcium carbonate accumulates on the floor as a stalamite or flowstone. It depends on how much calcium is in the water, how much carbon dioxide is in the water, the temperature of the water and the air, the pressure of the gas in solution and the pressure of the cave air. There isn't any hard and fast rule. A one half inch long stalactite can form fairly fast, in as little as ten years, or it can take millions of years. I have seen both.