this depends on what type of food you prefer, but if you are an asian you can spend 1800 - 3000 Qrs for food in a month.

for eating outside
breakfast - 10 to 20 Qrs
Lunch - 20- 35 Qrs
Dinner - 30 - 50 Qrs

but if you home cook, might spend only for 800-1200Qrs.

hope this will help you & gives you an idea. the cost of living here in Qatar is much high.