Usually rapid. Especially for mania - but I have had a few times when depression fell upon me as if someone had dropped a dark net over me. Depression is generally triggered by season and time of day. Mania...you know, good question. Either the pnset of mania triggers anger or anger triggers mania (with regard to an "irritable" mania). A "good" mania starts out as hypomania and can be triggered by season and oftentimes, music.
But sometimes it feels like moods just...happen. No rhyme or reason - especially true of mania.
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