OK, my normal brewing practice is to boil about 1 oz of hops, 1 lb DME in 1 gal of water for 60 mins. There are more steps (adding other ingredients, but not when making a tea), but that is the basis for extracting the bitterness from the hop.
Start off with 1/2 gal of water and 1 oz of hops.
You could also double up on the hop and only boil for 30 mins. It comes out the same. Any extra tea can be placed in a jar for a later use.
Either way, you have to have enough water left over that hasn't evaporated. Whatever water that does remain will have a high concentrate of bitterness in it. Sample a small bit and you'll pucker. I know I did.
You don't want to start with too much water, say 1 gal, because you'll end up diluting (thinning out) your beer and lowering the alcohol %. That's why I say to start with a 1/2 gal. first.
You can always add water later if need be...if you have too much water then you can boil longer to let some evaporate.