“How long after a petition gets approved do you have to pay the NVC?”
Once a petition’s approved by the USCIS, you’ll get an invoice from the NVC by email. It’s going to take a few weeks, could take up to a month before you get it. But, once you get it, you actually have up to a year. Obviously you don’t want to wait that long. And also read the letter you get very carefully, because these kinds of things are subject to change at any time. But, currently so of August, 2018, they’re still saying they’ll keep your case active for a year.
One important thing to know, is that the approval notice from the USCIS is only valid for four months, even though they’ll keep your case open for up to 12 months as long as you’re pursuing the visa. Just keep in mind if there’s any discrepancies, anything wrong with your application, they can always come back and deny you, because the validity period of the approval notice has expired. We’ve seen cases like that as well. So we highly recommend not sitting on these things for long. Move forward as soon as you can.