It depends on the size of your window air conditioner. A small 5,000 BTU air conditioner consumes around 600 Watts of electricity so you'd use 1,200 kwh in 2 hours. So the cost to run a 5,000 BTU air conditioner would cost you $1.20 per day if you ran your air conditioner for 20 hours a day.
$1.20 per day x 30 days would be $36.00 per month to run a small air conditioner. The larger ones can cost up to $100.00 a month to run.