At my hospital, we get lots of elderly pts from LTC or SNF's. If these pts have existing PICC lines, Central Lines, or Udall's we always draw one of the cultures from the line in case the infection has originated there. If the draw from the line comes back positive, the line is removed, and the tip is cultured to confirm the bacteremia. In a pt where they do not have an existing line, the cultures are drawn from 2 sites because like checking stool for C-Diff, you can get false readings. I have seen that generally, if the pt is afebrile, and not displaying signs of septicemia, but infection is suspected, the blood cultures are drawn from 2 sites to rule out false positives, since contamination can occur even with aseptic technique. The only times I see positive cultures as significant to our doctors is if both are positive, and they grow a bacteria that is not considered "normal" to the body, or if the indwelling line tests positive.
As to the time it takes to get results, it varies. I usually see preliminary reports at around 24 hours, which will state "growth" or "no growth" If there is growth, the determination of the type of bacteria, and what its sensitive to can take from 48-72 hours, at which point the MD will continue the same treatment, or change it if needed.
Most positives I have seen from blood are staph bacteria.