One that seems to be in fashion these days, although you never know how long the fashion lasts, is: internal uprisings that somehow these monarchical areas, when life got tough, the people must have risen up against them.

有一个理论现在很流行,虽然不知道流行了多久,那就是认为,这些王国中起义的内部原因是,当民生困苦时,人们就必然奋起反抗

耶鲁公开课 - 古希腊历史简介课程节选

I'm going to live here for, who knows how long. Quite a long time.

我打算定居在这儿，不知道多久。会是相当长一段时间。

追忆艾米 - SpeakingMax英语口语达人