Continuity mistake: In the evidence locker, Ziva opens a folder and the white notepad is on the left-hand side. Camera changes to behind her, and now the white pad is on the right. (00:26:40)
Continuity mistake: When we first see young Abby, the way she's holding her bear changes between shots. (00:04:50)
Continuity mistake: Abby says that McGee is the most helpful person with clipboards. When she does, the position of the clipboard in her arms is constantly changing between instant cuts. (00:03:30)
Factual error: There is a flashback scene in this episode, where a young, dejected Abby Sciuto is walking along railroad tracks after failing to give the bear back to Ricki. This scene starts with a shot of the railroad tracks, then pans up to Abby. It can be seen that the rails are laid on concrete sleepers, with modern steel clips holding the rails to the sleepers. Given that the flashback appears to take place when she was at least a couple of decades younger (i.e. the mid-1990s), such modern methods of construction wouldn't have been used. A period-accurate railroad track would have wooden sleepers, spikes and lighter rail. As such, it's obvious that the producers just went to the nearest railroad tracks and filmed the scene there.
Chosen answer: It is never stated, however she must have been there several years if she is on presidential detail.