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There's no way theologically to put an arbitrary TIME limit to it.

This is a textbook strawman. The limit is when all Papally appointed Bishops die. Bishops are Appointed or Consecrated around 35 and if they die around 80, that's around 45 years. There'll be outliers, so you can add 5 to 10 years at most. Beyond that is plainly ridiculous.

XavierSem, all these papally appointed bishops adhere to the heretical robber council. They lost their offices just like all the antipopes lost theirs or didn't attain any to begin with, for the very same reason. Your reasoning is confused. These antichrist "bishops" play no role at all.

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This sounds more like Home Aloneism than sedevacantism. There are certain home alone types who only go to Mass to priests who were ordained before Vatican 2, and who received jurisdiction over a parish before V2. Those people might think what is being described here. When those old priests die (and most of them already have in recent years), many of their parishioners become *actual* home aloners.
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No major sedevacantist group adheres to the view XavierSem is arguing against, namely that a new pope can only come from a bishop or cardinal appointed by P12 or J23, and in fact I don't think I've ever even run into a layman who believes that.
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This whole thread is basically XavierSem claiming sedevacantists believe something that they actually don't believe.

No, the whole thread is XavierSem saying that sedevacantists' theories are incoherent because they don't believe what he's saying, and they don't believe what he's saying because it would reveal their theories as incoherent.

To answer the main question here, of how sedevacantists think the Church can get a pope again, sedevacantists generally agree that if Francis (Bergoglio) were to repent and publicly abjure his errors, he would become a valid pope. Different schools of thought explain this differently, but they pretty much all agree on the basic idea. The people who adhere to the Thesis of Bp. Guerard des Lauriers would simply say that he was already validly elected, and simply removed the obstacle that was preventing him from being pope, so he forthwith becomes pope. Sedevacantists who do not adhere to the Thesis of Bp. Guerard would probably say Francis became pope by being accepted by the whole Church as pope. This is called election by "acclamation", and I believe it has happened in the past. It has been discussed by St. Robert Bellarmine and numerous other theologians.
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Alternatively, most sedevacantists would probably accept it if several Novus Ordo cardinals -- or possibly even bishops -- renounced their errors, denounced Bergoglio as a modernist heretic, declared the papacy vacant, and elected a pope after he refused to recant his errors. Sedevacantists would accept a pope resulting from this process for similar reasons to the prior scenario I mentioned.
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This is off the top of my head so some sedes might want to chime in and tinker with some of the details of this, but substantially that's the answer to your question. Note that it doesn't place any time limit on the Church. And yes, there are objections that can be made to these ideas, but they are of trifling difficulty in comparison with the difficulty of saying that real popes have been behind the universal heresy and apostasy of the Vatican 2 religion.