She can hold them as long as she wants....or until she's no longer the Speaker of the House. Of course holding on to them means she might as well use them to wipe her ass. Until they are officially given to the Senate, the articles are as useless as tits on a boar hog. In an impeachment process, the House fundamentally plays the role of prosecutor and the articles of impeachment are analogous to a prosecutor's indictment. If a prosecutor writes out an indictment but doesn't file it with the court, the accused really isn't functionally indicted and is in no legal jeopardy.
The Senate plays the role of the court who tries the case. Per the constitution, the Senate has the sole power to try the impeachment. Nancy and the House have zero authority to dictate any terms of the trial to the Senate just as a prosecutor in a criminal case doesn't get to dictate terms to the court. They can make motions to the court to request something be done such as calling witnesses. The defense also gets to make motions of their own and make counter arguments to the oppositions motions. These motions will get adjudicated by the court. Don't like the court's decision....tough *******. In a criminal court you could try to appeal and convince the appellate court of the merits of your motion. In an impeachment, the Senate is functionally the Supreme (and only) court. There's precedent to this....the whole witness issue played out in the Clinton impeachment. The House filed motions to call witnesses. Clinton's defense filed counter motions. The Senate ruled they would hear no live witness testimony.
If Nancy grows the balls to actually file her impeachment with the Senate, the same thing will happen. If she doesn't grow those balls, then Trump has a plausible argument that he really hasn't been impeached.