I think it's inevitable that the UK will rejoin the EU. However, I don't think that's going to happen any time soon.
For it to happen, first Labour must win the next election, which will probably be held in October next year.
If they win, as seems likely they will, Labour will first need to be seen to do their best to "make Brexit work" by taking a more realistic and pragmatic approach to relations with the EU. Then, come the time of the election after that -- presumably 2028 or 2029 -- they may be able to go into the election saying, "We've given it our best shot but Brexit simply isn't working. We need a new, closer, relationship with the EU, and if we win the election we'll take that as a mandate to start discussing that with them.
There's no point in speculating about what happens after that. As far as I'm concerned, re-joining is the only sensible option -- joining the single market, for example, without joining the EU, seems pointless because we wouldn't have any say in important decisions that affected key aspects of the economy -- but people will want to debate all the other possibilities, and we don't know what the political or economic situation will be like in the EU or the UK that far ahead.
Then, assuming the British government decides to apply to rejoin, and assuming the EU agree, that will need negotiating with the EU, which will take as long as it takes, and then the new terms of accession with need to be agreed by both the EU and all the then member states.
Meanwhile, there's the question of what happens to the UK Conservative Party, which is gearing up for big fight, assuming they lose the election, about where they go from here. At the moment, they're a very unhappy coalition between right-wing populists who previously supported UKIP and more traditional Conservatives, who Boris Johnson was able to bring together temporarily because they both wanted to break the parliamentary deadlock over Brexit and to keep Jeremy Corbyn out of Downing Street.
Now both those goals have been achieved, there's little to keep them together, and the coalition seems set to break apart after the next election, with results that are completely unpredictable.