Minecraft wiki entry for Pointed_Dripstone, associated text: When the uppermost block of a stalactite less than 11 blocks tall gets randomly ticked, it checks for a water source or waterlogged block two blocks above it and a cauldron within 10 blocks under the tip with no non-air blocks in between. If both conditions are satisfied, there is a 45⁄256 (~17.6%) chance for it to drip water and fill the cauldron by one level. If there is a lava source above the stalactite, there is a 15⁄256 (~5.9%) chance for it to completely fill an empty cauldron with lava instead, making lava a renewable resource. Refilling a cauldron with water or lava averages nearly one Minecraft day (19+ minutes) although the actual time for any individual refill varies.