1. Salt Urns: These urns are made from compressed sea salt and are designed to dissolve quickly in water. Typically, a salt urn will begin to dissolve within a few minutes of being placed in water and will be fully dissolved within a few hours, depending on the water's movement and temperature.

2. Paper Urns: Urns made from recycled paper or other paper products might take a bit longer to dissolve than salt urns. They usually start breaking down within minutes but can take several hours to fully dissolve.

3. Gelatin Urns: These are made from a mixture that includes gelatin, which dissolves in water over time. Like salt urns, they'll begin to break down within a few minutes and can take a few hours to completely dissolve.

4. Other Natural Materials: Urns made from materials like sand, compacted plant fibers, or other organic compounds can vary in their dissolution times. Some might take several hours, while others could take longer.

Several factors can influence the dissolution rate of a biodegradable urn in water:

- Water Temperature: Colder water slows down the dissolution process, while warmer water speeds it up.

- Water Movement: Urns placed in still water (like a calm lake) will dissolve slower than in moving water (like a river or the ocean).

- Urn Thickness: The overall thickness and density of the urn can affect how quickly it breaks down in water.

- Placement: If the urn is anchored or weighted down, it might take longer to break down than if it's freely floating.

If you're considering a water burial with a biodegradable urn, it's essential to check local regulations and guidelines. Some areas have restrictions or guidelines for water burials to protect the environment and aquatic ecosystems.