In the heat, keep the grass tall (at least 4″) and give it a full 1″ of water every 5 days or so.  Everyone has different water pressure and sprinklers so the only way to know how long it takes to put 1″ of water on is to measure it.  Get a shallow, straight sided can like a cat food tin or tuna fish can and time how long it takes to fill it to an inch. You could also use a rain gauge if you have one.  Note how long it took and water every five days for that long.