It's all hypothetical at this point because the timeline is constantly changing. Even if they win, Labour will just keep blocking every new deal in Parliament until the EU eventually just tells the UK to piss off, and it ends up being a hard Brexit - which, strangely, is what I think they (Labour) want? It would give them a few years of being able to say "I told you so!" and would then guarantee them the next election anyway. Under the circumstances, I'd say a year. Unhindered, potentially much sooner.
If Labour win, they will end Brexit almost immediately, probably through the guise of a second referendum to make it seem democratic (bearing in mind that there are many people out there who are still neutral, and a growing number like me who are just over it already and maybe wouldn't vote for or against it a second time round). And then everything would go back to "normal", except the EU won't just let this slide. They will do their utmost to make an example out of the UK so that no other country in the bloc ever dares attempt anything like this ever again.
So, you're damned if you do, damned if you don't.