I don't think anybody knows. If you were thinking more like a convention on something where people could just refuse to sign and come back in a few years. No.
Officially the negotiation period was entered into some time back (note that this is not the same as the referendum --
https://www.bbc.co.uk/news/uk-politics-39143978 ), the end of which the member state exits regardless. This is what the looming deadline thing is all about as that is rapidly approaching (March next year*), and what this talk about the backstop agreement (the agreement that would go into place if the deadline passes and no final deal is agreed upon) concerns --
https://www.express.co.uk/news/poli...an-Brexit-backstop-meaning-backstop-explained .
*of course we also have Christmas and February break before then, and Easter right after, which means about a month less than that as far as time goes.
https://www.parliament.uk/about/faqs/house-of-commons-faqs/business-faq-page/recess-dates/
Christmas 20 December 2018 7 January 2019
February 14 February 2019 25 February 2019
I don't know offhand what it would take as far as member state votes to prolong talks if that would be the will of the other EU states. While there is talk of a bit of leeway I am not seeing much of anything as far as prolonging talks.
I believe the final deal also has to be voted on by parliament as well (there was a court decision on what needs to go there some years ago, as well as one to even trigger article 50).
https://www.theguardian.com/politic...-may-postpones-brexit-deal-meaningful-vote-eu
Said article 50 also saw some challenges a little while back
https://www.shropshirestar.com/news...enge-to-brexit-and-what-is-westminsters-view/
There are no elections likely to happen before the upcoming deadline, and the thing emigre mentioned about the leadership contest (the negotiations, for something generally considered to one of the defining events of probably this and several decades to come, are going poorly to say the least and is pleasing basically nobody at this point**, which meant the prime minister risked being ousted by her own party) means we are not likely to see anything happen on that front.
https://www.ft.com/content/eaeac920-feb1-11e8-aebf-99e208d3e521
I don't know what would happen if somehow there was an election (one that happened before the deadline) with a party elected that would want to stop it and what they could do. Such a thing would be purely hypothetical though and is not really worth considering.
**there was no official plan at the time of the vote, none by the time article 50 was triggered, and we have still not seen one that everybody agrees on (the first thing to come close to anything was the widely maligned Chequers deal, one that any fool could have seen would have been, in July 2018). Generally speaking there are three camps, though obviously there are people within any one of them that have things they will and will not live with as far as laws, N. Ireland-ROI border, finance....
Said camps are hard, soft and remain. Hard essentially wants to be another country in the world (some like the phrase clean break), soft would probably see the UK remain with one foot in the EU (similar to how Switzerland, Norway and such by virtue of being able to be driven to from everywhere else in Europe reckon it is worth being able to trade easily with them and no have to worry about competing standards, tariffs and such) and remain would likely seek to cancel the lot and go from there. Generally the European negotiators favour the latter two options, UK members of parliament and other figures don't have enough of a majority to do anything outright (for a leadership confidence contest to be called, and for that many votes against them, is not a trivial thing). In case you were curious the US, or at least Trump (
https://www.bbc.co.uk/news/uk-politics-46352463 ), would appear to be more in the hard camp (talks of trade deals say they want the UK out of the EU single market, something only likely to happen with the hard scenario) and that is no small thing --
https://www.ons.gov.uk/businessindu...ade/articles/whodoestheuktradewith/2017-02-21 .
So I would go with it would take something quite special for it to be outright stopped (and at this point it would likely have to come from the EU side of things). Anybody's guess as to what goes as far as a backstop deal and whether there will be one in place, much less a final deal. If nothing is agreed upon and there is no backstop it becomes a situation known as no-deal.
https://www.itv.com/news/2018-12-13/how-will-no-deal-brexit-affect-me/