Indian telcos have started deactivating SIM cards that aren't active ([news coverage][1]), and they no longer consider account balance for this, so all of the above answers are no longer valid.

To keep the SIM active, you have to keep recharging it to extend validity. Every network has cheap plans specifically for this. 
Airtel has a INR 23(€0.3) plan that extends validity by 28 days. Other networks like Vodafone have similar plans (INR 35 or €0.4).

You can recharge from websites like Paytm or Freecharge.

A more convenient way is to install the mobile app of your network (Airtel's app is called, well, MyAirtel) and login with your phone number. It shows you remaining validity and even sends a notification when it is about to expire, and you can recharge from the same app.


  [1]: https://www.zeebiz.com/india/news-you-will-lose-your-sim-connection-soon-airtel-idea-vodafone-plan-this-big-move-know-why-what-you-should-do-72788