Instead of finding out what others spend - that can vary by the size of the family, the area of the country, their eating habits, and a host of other things - I would start by trying to figure out what the average that you've spent the last few months. It doesn't need to be exact, if you don't have convenient receipts, just do the best you can.
Then, if you are interested in trimming you spending in that area, set it just 10% less to start with. If you are OK with what you are spending, just leave it be.
Once that number is set, track things for a few months to see if that number is really right. Adjust it - up or down - if that initial number turns out to not really be right. The initial point is to just really understand what you are spending.
Once you feel you have a good hold on what you are really spending, if you are trying to trim you costs more, trim a little at a time from your budget until you think you've gone as low as you can. Trying to trim radically all at once usually doesn't work well - you need to tackle things a little at a time.
Budgets take awhile to fine tune for your individual situation. And they are never "done"
- tweaking is constantly needed. Something is always happening that causes you to need to revise them: prices go up/down, your situation changes, you realize you left something out that needed to be there, you decide to cut lower in one category to up another, etc.