I am travelling to Jeddah from Kolkata on the 15th of June via Doha. I have a layover in Doha of 18 hours, 15 min. Should I get accommodations in Doha by the airline since my next connecting flight is more then 6 hours? If there is a law that passengers should get accommodations if the connecting flight is more then 6 hours, please let me know.
I have never heard of a law or regulation that would require an airline to provide transit accommodations based on a scheduled connection (as opposed to a cancellation or other operational failure, for example). After all, you are the one choosing to accept the itinerary as given.
But it is true that such accommodations are written into some fare rules on some airlines. The travel industry term is Stopover Paid By Carrier (STPB), a web search on which will be informative. On this site, also see Does a layover of more than 6 hours entitle me to accommodation by the airline?
In the specific case of Qatar Airways, a simple web search indicates that it is possible:
What if I have a transit time of over 8 hours in Doha?
Qatar Airways may offer transit hotel accommodation for customers who face extended transit times (between 8 and 24 hours) in Doha, en-route to their final destination depending on the fare and route purchased.
You should consult the airline's Conditions of Carriage as well as the rules of your fare first, then contact the airline directly to see if you qualify. On a flight as short as DOH-CCU, I doubt that will be the case, but it doesn't hurt to ask.
For some other airlines which frequently offer accommodations on long layovers, see the March 2015 blog article "Long Layover Ahead of You? These Airlines Will Hook You Up With Free Hotel" at travel site MapHappy.