The six-month rule is the global default worth planning around: many countries require your passport to remain valid for six months beyond your arrival or departure date, and airlines enforce this at check-in, refusing boarding even when the destination might have been flexible. A smaller group requires only three months beyond departure, and some accept validity for just the length of stay, but you rarely lose anything by treating six months as the standard.
Blank pages are the forgotten cousin: many countries require one or two completely blank visa pages for stamps, and running out can genuinely get you turned away. Frequent travellers on nearly full passports should renew early rather than gamble.
A practical rule: once a passport has less than nine months of validity or fewer than four blank pages, start the renewal, since processing takes time and trips appear suddenly. Verify the specific requirement for your destination on its embassy or official immigration site, because exceptions run in both directions.