How often do you add cow dung manure to houseplants?
First thing is to understand that cow dung, or any kind of manure, has to be composted before it can be used for plants. https://www.gardeningknowhow.com/composting/manures/cow-manure-compost.htm.
You can purchase composted cow manure, if you don't have enough plants to warrant your own compost pile.
Most people don't use composts or manures as regular fertilizer for houseplants, although it can be mixed in with the soil when repotting which for most plants should be done once a year.
Here's more info on fertilizing houseplants: https://www.gardeningknowhow.com/houseplants/hpgen/signs-of-over-fertilization-in-houseplants.htm