For the longest time is a commonly used phrase that refers to a significant duration of time or a period that feels like it is going on forever. Synonyms that can be used as replacements for this phrase include 'for ages', 'for a prolonged period', 'for an extended time', 'for an eternity', 'for a significant interval', 'for a protracted duration', 'for much time' and 'for quite a while'. All these synonyms are useful when there is a need to convey the idea of something lasting a long time. Using any of these synonyms will help to diversify one's vocabulary and bring a fresh perspective to a piece of writing or conversation.