It's a good question. I looked into it some time ago, but didn't find much to go on. I found this: Curr Opin Cardiol. 2010 Jan;25(1):1-7. doi: 10.1097/HCO.0b013e3283336d52.
Persistent atrial fibrillation: current approach and controversies.
Parkash R1, Verma A, Tang AS.
which may help - I can't access it, as my Athens login doesn't work. They discuss definition in the abstract. The NICE AF guideline uses this term, they may reference it perhaps - but you'll have tried that no doubt!
PubMed Title search goes back to the 50's, but the term is only common in the title of papers in the late 90's so that is a clue perhaps.
I think the 7-day cut-off was introduced without a specific rationale by a paper by Samuel Levy in 1995 (J Cardiovasc Electrophysiol, 6:69-74), see also