I have a game where I have to get data from the server (through REST WebService with JSON) but the problem is I don't know when the data will be available on the server. So, I decided to use such a method that hit Server after specific time or on request on every frame of the game. But certainly this is not the right, scale able and efficient approach. Obviously, hammering is not the right choice. 
Now my question is that how do I know that data has arrived at server so that I can use this data to run my game. Or how should I direct the back-end team to design the server in an efficient way that it responds efficiently. 
Remember at server side I have Python while client side is C# with unity game-engine.
It is clearly difficult to provide an answer with little details. TL;DR is that it depends on what game you are developing. However, polling is very inefficient for at least three reasons:

The former, as you have already pointed out, it is inefficient because you generate additional workload when there is no need
The latter, because it requires TCP - server-generated updates can be sent using UDP instead, with some pros and cons (like potential loss of packets due to lack of ACK)
You may get the updates too late, particularly in the case of multiplayer games. Imagine that the last update happened right after the previous poll, and your poll is each 5 seconds. The status could be already stale.

The long and the short of it is that if you are developing a turn-based game, poll could be alright. If you are developing (as the use of Unity3D would suggest) a real-time game, then server-generated updates, ideally using UDP, are in my opinion the way to go.
Hope that helps and good luck with your project.
Check the data has updated at server without requesting every frame of the game