Truman cells can last several months before needing to be cleaned, but actual time will vary depending on how dirty the air is. You will probably be able to tell if the cell needs to be cleaned by listening to it. Dirty cells are noisier than clean ones, and will often make popping or creaking noises. This is a sign that you should remove the Truman cell unit and wash it. Oreck will also usually include a schedule with the air purifier that will show you specific dates, depending on when you start using the purifier, for cleaning its various filters. Most Truman cells are locked down by two simple latches you will need to unscrew. Lift the cell up to detach it from the purifier.
This is a good time to check the other filters as well, especially if you do not use a cleaning schedule chart. The catalytic ozone converter should be periodically vacuumed to keep it in prime working condition, and the grills can be brushed up with a rag or brush (Oreck usually provides a cleaning brush, too).
When you have removed the Truman cell, you have two cleaning options to choose from. The least expensive option is to use warm water or a warm water solution with a small amount of liquid detergent added. Soak the filter for a couple hours, then wipe it down and allow it to fully dry before putting it back into the purifier. Any water left could damage other parts of the system.
If you want to use the more expensive and official cleaning procedure, then you should buy a bottle of Truman cell cleaner from Oreck, which is used to dissolved any build-up on the cell panels for easier cleaning. This method takes less time and uses less water, which means you will not have to wait as long for the unit to dry, but you will need to keep ordering bottles from Oreck.