Basically on tues, Teresa mays vote will go back to parliament for a third meaningful vote.
If it is passed, she will seek an extension of three months from the Eu on thurs and Friday at the summit, to allow time for it to be passed into legislation.
If it is rejected again, she will seek an extension of likely two years from the eu on Thursday and Friday at the summit, and we will then be looking at many different options, from a whole new strategy, to leadership contests, general election, or second referendum,
So it's either three months or two years depending on what happens at next weeks meaningful vote.
The Eu will definetly grant the three month extension, it is not absolute they will for thr two years, but tusk has said he will urge it and is open to it, so it's highly likely it will be granted.
She will then amend the legislation on the 25th based on whatever is decided,