There are a mix of standards in play.
As discussed in Immigration Rules paragraph 20: "If the leave was given for a period of six months or less or conferred by a visit visa", then it "lapses on his going to a country or territory outside the common travel area".
Under article 13(4) of the 2000 Order, for an absent limited leave holder whose leave expires before the holder can complete a continuous period of two years outside the UK, the leave "shall remain in force ... until the date on which it would otherwise have expired".
Otherwise, under article 13(4)(a), the general lapse is after a continuous period of two years outside the UK.
However, a variety of exceptions exist, such as:
Also, Immigration Rules paragraph 19 allows a person with ILR who has been outside the UK for more than two years to apply for ILE entry clearance if he "can demonstrate ... strong ties to the United Kingdom and intends to make [it] his permanent home."
Immigration Rules paragraph 20 apparently provides a minimal safety net of sorts through reconsideration at the border for possible eligibility, for those whose lease has lapsed by remaining outside the UK:
A person whose leave has lapsed and who returns after a temporary absence abroad within the period of this earlier leave has no claim to admission as a returning resident. His application to re-enter the United Kingdom should be considered in the light of all the relevant circumstances. The same time limit and any conditions attached will normally be reimposed if he meets the requirements of these Rules, unless he is seeking admission in a different capacity from the one in which he was last given leave to enter or remain.
Of course, that's only a broad overview of applicable paragraphs and someone like OP should consult with an immigration solicitor about taking action after possible lapse.