Shaival Gupta wrote:

How much time should I allow to pass after the weed killer application
to lay down the sod ?

The friendly folks at Home Depot told me you only have to wait
48 hours after applying Roundup before laying sod. I think
I'd wait a little longer than that (a week?) to be sure, but
you definitely have to do something to get rid of the weeds
before laying the sod.

BTW, they also told me that contrary to advice posted here, it
is a bad idea to lay sod before the end of March or so,
because the grass is still dormant and will not begin to grow
until then anyway. And make sure you rent a sod roller or
your sod will surely fail.