Looked at my natural gas usage. In the summer it only used for water heating/keeping it hot at 10 therms/month. Total annual gas usage is 700+ therms. 12*10/700 = 17%. And this includes both heating and keeping it hot.

This site [1] quotes 0.0113 therms to heat a gallon of water. 120 therms / 0.0113 / 365 = 29 gals of hot water per day. This place [2] estimates 41 gallons of hot water per day used during peak demand. My estimate is that we use more for showers, less for washers/dishwashers, same for faucets. Looks like I would spend all of my BTUs on heating these 29 gals of water every day. Most likely close to single digits %% for keeping it hot.

[1]https://webbsupplycompany.com/scalesafe/what-does-it-cost-to...

[2]https://michaelandson.com/blog/what-size-water-heater-do-i-n...