Reusable air filters require more than just water to be cleaned! Most manufacturers of reusable filters also manufacture the chemicals necessary to clean them, and are sold at most auto parts stores. Apply this chemical to the filter's surface area (which is the paper mesh area the air flows through) on both sides and allow about 15 minutes of soaking time.
After that time has passed, rinse it clean with water, gently (forceful spraying can cause damage to the filter). Gently shake off the excess water, and allow the rest of the water to evaporate naturally (blow-drying it may damage it in the same way spraying water on the filter would).
The final step is to apply the air filter oil (also sold in most auto parts stores). It is important to not over-oil the filter, because some oils damage the mass air flow sensors on the cars of some manufacuturers, which will eventually cause vehicles that depend on the mass air flow sensor's information to run poorly until a replacement is installed. A damaged mass air flow sensor can cause some vehicles to run so poorly, that other parts get damaged as well over time.
After the filter has been properly oiled, it is ready to be installed and keep the engine's air filtered for thousands of miles! The time between cleanings depends on what environment you are filtering.
K&N Cotton Air Filter Cleaning & Re-Oiling
K&N's video explains specifically how to clean the filters they produce. As you can see from the video, they have their own cleaning and oil products, and different waiting times. If you have to clean another brand's filter, you should check their instructions for brand-specific details as well.
The reusable air filter pictured above is clean, oiled, and ready to begin filtering air contaminants! Reusable air filters don't need to be replaced, and sometimes improve an engine's performance as long as they're kept clean.