Its 30 days from the date and time of activation. You can check your usage by sms. --> ACT MYDATA to 111.
For karnataka circle you can activate it by *444*x# where x = 4,16,95
Vodafone guys are pretty serious about their cust care these days. Although its super hard to actually get a cust care rep on the phone... The whole ivrs maze n all. They are very responsive via twitter, though.