Not amazing at all, the length of time depends on how similar the ambitions of both sides are to what is already in place. Labour would want an arrangement as close as possible to single market membership, most probably the only sticking point being free movement of people but even then free movement, when all the right wing rhetoric on immigrants is removed, is a massive positive for the normal priorities of any sensible government (the economy, tax receipts, the NHS and pretty much everything else you care to think of except perhaps education) so they'd probably even get behind that eventually.