They will keep them until it is clear to everyone north of the equator that they can't cut it in the NFL. When the Skins FO makes a commitment to a player - by spending a high draft pick on them or signing them to a big deal - they stick by that player longer than an objective observer would.
There are loads of examples here...
Here is when you will know that the Skins are preparing to cut one of these guys loose:
You start to read that the player is not paying attention at meetings or that he has rubbed some of his teammates the wrong way or that he hasn't committed himself fully to the team philosophy.
That is the prelude to separating the player from the team. Until then...