1st grade teacher for our son, age 6 just sent home in her weekly note saying....
your child needs to read daily, work on spelling words, work on shape and number wordsand do a few addition problems... I think 15 minutes a day would be enough time. Alot of this can be done orally, while they are riding in the car, etc. We don't want to burn them out!
Last year in K our older son would get reall tiredand bored after about 10 minutes but gradually the time span increased, probably never went more than 25 minutes for him to do anything the teacher sent home for him to do. If he dwadled longer than 1/ hour and was having a real difficult time keeping focused I was willing to write a note to the teacher sending in what he had gotten done and tell her to send it back and we would finish it the next night... know that wouldn't work with all teachers and not the best for higer grade levels...
This isn't 'homework' like you are asking for but we do at home speech therapy with both boys.... and here's what the attention span is for that
big mister, 6 can stand about 10-15 minutes of drill... show a picture, he says the word or you say a word and he repeats it. If you throw in a game (board, card, anything... somthing with toys) and say the words between taking turns then he can last about an hour, much of that time is taken up with playing though.
My younger son, ae 4.5 LOVES drill, We have been working with him since age 2 and his time span for drill is probably about 30-35 minutes (you have to goof up some during that time with him) but he's 'with you' the whole time and actually getting something from it, add a board game or something to his and he can last a little longer, but actual work time decreases. He's hearing impared and although he's aided I think having someone directly in front of and speaking clearly and slowly to him helps keep him with you if that makes sense.