My tank got down to 1/4 of itself during the summer 2 or 3 years ago. And then I went easy with it. I use it for top-up watering in my garden.
On the dry restrictions, I was allowed to water on Tuesday morning and Saturday morning with a hose.
In summer, that's not enough if it gets super hot. So, I was using the tank on Thursdays as well; just to give the garden the extra drink on the hot weeks/days.
My best friend installed 5,000 litres (more space for the tanks in her place). She uses the water for one toilet, her washing machine and to water her garden. They have yet to run dry. She probably does about 4 or 5 loads a week? possibly more?