Depends on what you're using it for. For cleaning, I use about 1-2 scoops per 5 gallons of water, which is actually more than I need. For delabeling, I use about 4 scoops in a bathtub full of water, and I have no idea how much water my tub holds.
I tend to use 1/3 to 1/2 scoop per 5 gallons hot water, and that works great for me when cleaning carboys with a soak for a few hours. When removing bottle labels, I use more of 1/2 to 1 scoop per 5 gallons.
It may seem wasteful to rinse a carboy, Better Bottle or bucket five times, but once the yeast and hop particles are gone, you can collect the rinse water in a bucket and pour it in your washing machine to do laundry - it's perfectly fine for that and saves a ton of water.
I reuse most of my brewing waste water for laundry, filling toilet tanks, watering plants, etc.