The information from the team at British in Europe is that “It is sufficient to come back for just a few days.” The European Commission are due to publish guidelines on this issue, but have not done so yet! The British in Europe team have seen a draft.
Just in case, here is the Case precedent which involves someone with a Long Term resident EU permit – https://curia.europa.eu/juris/document/document.jsf;jsessionid=C181B47DB18EE32CB80FCC99D8A4D945?text=&docid=252448&pageIndex=0&doclang=en&mode=lst&dir=&occ=first&part=1&cid=5212406
