You can't really change that time as it depends on the ISP's throughout the world and how fast they update their lists. If it is taking really long on your end you can change your hosts file or use something like opendns at home. I use it and like it very much.
If you have waited that long there are several possibilities:
The registrar hasn't actually submitted your updated name server records (we had this once, also they forgot to actually register the domain).
The second possibility is that your DNS servers are set up incorrectly/have incorrect zones.
I'd go to your registrar first and see what's happening with them.
If .co.in domains are anything like .co domains, you may be in for some work as we spent 3 days working with a .co and custom DNS through NameCheap and Name.com and there was zero resolution. It's as if the domain simply refused to propagate the newly created DNS servers and this is far from the first time I've setup DNS / name servers. We even touched base with both support teams and they couldn't provide a reason as to why this was happening, not even a wild guess.
We finally ditched the .co and went with another extension, though I've had similar issues with other such domain extensions including .at and .in.