I depend on a number of things I think. How aggressive you boil, humidity, kettle shape, and surface area...
There are several calculators out there that will help you with this. Boil off time is the time it takes for the water to evaporate or how much is evaporated over a period of time.
What I did, was Put in 1 Gallon of water in my pot. I then Took a Yardstick or ruler, dipped it in the pot and measure the water.
I then went to the calculator and put the diameter of the pot and how deep it was and then I put in Inches how much water I had.
Then I boiled for 15 minutes. I measured the water again with the ruler or yardstick I input that number.
Calculated it and it told me what my Boil off was. You can do 15 minutes times 4 I think it is, or 30 minutes time 2, and so on. I just did the 15 minutes.
All in all you need to know how much water evaporates during your boil time because you'll need to add that in the pot before you start the boil so you can anticipate how much water you'll need. And if I recall, I think it's around 1 to 2 quarts (Half Gallon) you'll need on average.
So with a 1 Gallon recipe (IF IT'S NOT ALL GRAIN), you'll need around 1.5 gallons of water for a 1-gallon batch.
If it's an All Grain batch, you'll need half a gallon for boil off. Then you'll need around 1.5 quarts per 1lb of grain you use for a 1-gallon batch.