The USCIS guidelines indicate that after the I-130 forms for priority (non-immediate relative) cases are adjudicated, they are sent to the NVC. NVC will send the petitioner a letter with a case number and will say, unfortunately visa numbers are not yet available in your category. They will advise to monitor the Visa Bulletin. When priority dates get within a year, I believe, they will send a fee bill. You should consider calling them at least once a year to keep the case current, and give them updates on your address and e-mail.
The priority date is set when the I-130 petition is filed. You can check the processing time for the I-130 adjudication by visiting www.uscis.gov. It may take a couple of years before the petition is adjudicated especially when the waiting time for the priority date is long. Once the case is approved, the file is forwarded to the National Visa Center. When the priority date is reachd (or about to be reached), consular processing begins.
In some situations, the lengthy wait for the adjudication of the I-130 might work to the advatnage of families with children age out (reach 21) in the process and may need to invoke the Child Status Protection Act (CSPA) to qualify.
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It gets approved when CIS gets around to approving it. You are correct that you wait for the priority date to appear on the visa bulletin. Once the I-130 is approved, CIS generally notifies the NVC who will then be in contact with the petitioner when the visa number is close to becoming current.