Water diamonds, also known as hexagonal ice crystals, typically form within hours to a few days. The exact duration depends on various factors such as temperature, humidity, and atmospheric pressure. In general, under ideal conditions, water diamonds can form within a few hours.

The formation of water diamonds is a complex process that involves the crystallization of water molecules under specific temperature and pressure conditions. When the temperature drops below freezing and the humidity is high, water molecules start to arrange themselves into a hexagonal lattice structure, forming ice crystals. This process can occur rapidly, often within a few hours, especially in cold and humid environments.

According to research conducted by the National Snow and Ice Data Center, the formation of water diamonds can be influenced by several factors. For instance, a study published in the Journal of Geophysical Research: Atmospheres found that the formation of ice crystals in the atmosphere can occur within hours when the temperature is below -40 degrees Celsius and the relative humidity is above 90%. In such conditions, water molecules can rapidly freeze and form ice crystals, including water diamonds.

Moreover, the formation of water diamonds can also be influenced by the presence of impurities in the water. Impurities such as dust particles or other substances can act as nucleation sites, facilitating the formation of ice crystals. In these cases, the formation process may be slightly slower, taking a few days to complete.

In conclusion, the formation of water diamonds typically takes a few hours to a few days, depending on various environmental factors such as temperature, humidity, and the presence of impurities.