after parents get visa

[preet4]

I have applied for my parents to come here in U.S. My question is when they get the visa in 6 to 8 months(hopefully), do they have to come here right away or can they stay in India for a couple of months (to attend family marriage etc.) before coming here?
I have got the approval notice for both of them saying their case has been forwarded to NVC in NH and my parents will get something in mail at some point (something along those lines), I wonder how long that will take. Their priority date is Nov. 08.
thanks in advance for any response

[PraetorianXI]

Once their immigrant visa is approved. They have 6 months to use it to move permanently to the USA.

[preet4]

Once their immigrant visa is approved. They have 6 months to use it to move permanently to the USA.

Thanks for ur response It helps