It depends on the visa classification your petition is under. All the family based categories are oversubscribed, so there is a wait for a visa number to become available. Visas are issued first come, first served unless the annual limit based on country of birth is reached.The process generally has been improved to eliminate processsing waits so that the priority date wait is the only one that one will have. To achieve this, NVC will begin initial processing for an immigrant visa about two years before a petition is expected to become current. In the Visa Bulletin, there are two charts - one for final action and one for processing. The processing date is the priority date for which petitions with a date equal to or earlier can begin initial visa processing.Now this two year or so processing period is an estimate. Some cases will take longer, some shorter. It would appear that your case may have processed quickly and is ready for conprocessing and an interview. How quickly this happens depends on the post's workload and how close your priority date is to the final action date in the bulletin.