I doubt it would happen in Labour's first term, it would suck up all of their time and energy and give the pro-Brexit crowd endless fodder to attack them with. It could be a second term project, or maybe even a risky re-election bid if they perceive that it might win them enough votes to secure another term.

This is complicated by the fact that the whole process of re-joining would likely take longer than a single term to complete, and the EU is unlikely to start that process until they believe that support for it in the UK is strong enough that the whole thing won't just be abandoned part way through due to a change of policy or party in government.