I am using a Hercules MTB thriller,I drive a maximum of 75 km per week. When should I schedule periodic, quarterly full service for the bike?
On the bike you have, just keep the chain oiled and the brakes tight.
There is no point in bothering to service anything else, as regardless of what you do it will just drop off one fine day. In any case, single speed commuters don't need much maintenance.
The bike has really poor build quality.
Go to your bike shop to get stuff fixed when you have real problems.
As it is you'll have to throw away the bike in about 3 years(if you use it as much as you claim).
To everyone else reading the answer, the bike in question is this. A $75(USD) commuter BSO.