That would all depend on Your standard of living and what your used to! if you like living in dirty areas with what i would call Not so nice Surrounding then 4500 QR is about the going rate.
If you want more upper class living, with a mix of nationalities you can expect to pay from 10.000 QR upwards!
I have a husband, and 2 kids and i spend 600 to 800 PER WEEK on food shopping! So again its all down to your standard of living before you come to Qatar.