Arduino with wifi shield and 5AA batteries current draw Arduino uno with official Wifi Shield take 5 Volts. 
Arduino lights up as it should when powered but the WiFi shield is dead the l9 doesn't light up but works fine when arduino is connected to usb cable.

I use 5 battery holder 5x1.5V = 7.5 V ; 2000 mAH

50 mA - Arduino 
300 mA - WiFi shield

350 mA * 5h = 1750 mAh

5xAA = 7.5V ; 2000 mAh


Am I missing here something? It should last in the worst case scenario 5 hours with new batteries at 2000 mAH (which I think they have more but I do not have them on me right now).
As a general answer, yes. AA alkaline should be more around 2500~3000 mAh depending on quality. 5x AA in series would result in 7.5 Volts and 2500 mAh.

But that's only for almost fully charged batteries. AA batteries can range from 1.6V to 0.8V "dead for most purposes". As they drain, the voltage drops. 

What may be happening is that you are powering the Arduino through its power jack, which goes through a diode, causing a 0.7V drop. Then it goes to the linear regulator which has a very high drop out voltage. Or even via the VIN pin, bypassing the diode. But as the battery drains, the voltage drops below the regulator ' s limit.

The Arduino microcontroller can easily work below 5V. The shield might not.