Anywhere from 30-55 days. However, I don't think the embassy contacted the NVC at all in January. Last year, they reported openings on 2/15 and the first person that got an interview only had 17 days to plan for it.
you can change your appt date but beware it might make a long delay in getting another one.
where is your timeline? what day were you document ready approved and has an appt been scheduled since?