It depends on how their case was resolved, and the difference between the two paths is significant.
If the person went to trial and was convicted, they were already in the federal system before sentencing ever happened. The case was active, the paperwork was moving, and updating their designation to reflect a new facility assignment is relatively quick. You might see them appear in the Bureau of Prisons inmate locator within a few days of their arrival at the designated facility.
If they took a plea, the timeline stretches out. Plea cases involve a different administrative process on the backend, and the paperwork moving from the court to the BOP and then through the designation and transport system takes longer to work its way through. In those cases it can be anywhere from a few days to a couple of weeks before they show up in the locator with a confirmed facility assignment.
There is also a gap that happens during transport itself. When an inmate is in transit between facilities, whether moving through a regional transfer center or being transported by the US Marshals, they often temporarily disappear from or show limited information in the public-facing database. That is normal and does not mean something is wrong.
The BOP inmate locator at bop.gov is the most reliable place to search. If your person is not showing up yet, check back every day or two. If two weeks have passed since sentencing or arrival and there is still nothing, contacting the court clerk or the US Marshals office that handled the transport can help fill in the gap.