Do we know how long they sat there?
No. The article does not mention how long the packages sat before being discovered.
From the article:
K-9 cops are on edge over a "suspicious packages pattern" encountered between the last week of March and the first week of April.
"During the past week and a half, the K-9 Unit has responded to approximately five calls for unattended/suspicious packages which they cleared…
…all cases involved empty suitcases.
… cases appear … unusual … most cases of an unattended bag/package there is something in the container…
At Penn Station, a beat-up suitcase was found in the main waiting room at 9:40 a.m. on April 1, sources said.
The bulletin, sent to New York State Police and Westchester County cops on April 5, asked them to report "any similar cases" …
Another empty suitcase turned up at 26 Federal Plaza, sources said. Officials have not ruled out the chance the bags were left by pranksters or derelicts.