This plans offer 2 GB of data at normal speed but beyond 2 GB usage you will get speed of 40KBPS, so it means this plan comes with a FUP (fair usage point) limit. The plan is valid for 30 days but once you reach 2GB limit then your speed will be brought down to 40 KBPS.
There are some other players like AirCel is offering unlimited data plan for Rs. 98 with no cap limit. Some other players too offer cheap unlimited data plan. AirTel should look in market and launch its plans.