It'll take at least that long for the EU member states to forgive the UK for its fuckery. The memory of Brexit will have to fade enough in their minds before it's even considered.
It's doubtful that the same deal will be on the table, as it would be politically untenable domestically.
Getting France and Germany on board will be hard, given that they enjoy much more power in our absence.
The risk of our exit again when our xenophobia acts up would have to be objectively low, or no member state would take the chance on approval lest we fuck over their economy again when we throw an egocentric racist tantrum.
I expect (at least) one party will eventually adopt Rejoin as a distinguishing policy, and maybe sooner rather later.
But the appetite for Rejoin will probably depend on the shape of the UK economy and the political direction of the EU in 10+ years. If the Starmer project really has been delivering tangible growth by then, people may feel Brexit has (inadvertently) “worked” in the end. If the EU achieves greater and greater integration in the UK’s absence it may seem less palatable to enough voters.
Both of those are also going to be influenced by external factors like the direction of a possible Trump second term, the outcome of the war in Ukraine etc.
I would hypothesise now that the UK has left, France's proposals for a closer integrated EU standing army and two-speed EU are much more likely to go ahead.
Because of that I see a future in 20 years for something like a three-speed EU:
Full integration.
Free movement of goods and people.
Mutual recognition of qualifications and frictionless framework for EU standard goods back and forth across the boarder. With a seat but no voting rights for the discussion of said standards.
Scotland never wanted to leave in the first place. I believe there's strength in sticking with the other EU members (esp as I currently live in the US and feel the differences between US legislation and EU legislation most days e.g. right to repair) and I believe a lot of what people were sold on (particularly farmers and fishermen) to vote leave was inaccurate.
I hope so. Though 15 years may be a bit optimistic. The UK has passed through the phase of leaving the EU being a benefit in itself (freeing it up to rule the waves once again unfettered by the whims of politically-correct vino-drinking bureaucrats and such), and, faced with the sunk cost of its folly, has retreated into denial. Brexit may have cost us dearly, it goes, but it’s a price we have to pay to be true to our destiny, rather than pretending to be just another small country interchangeable with Spaniards and Belgians. Eventually the fit of pique will end and the consensus will settle on Brexit being a bit shite, and there not being any meaningful glorious destiny for which it is a price worth paying, and the question is how do we become like the Spaniards or Belgians (or, indeed, the Irish), enjoying the conveniences of the EU. It may take a generation though.