My instinct about this says that to the extent that they're used as relative pronouns, "when" implies more of a single moment, whereas "in which" implies that the timeframe has a duration. Another way to think of it might be that "when" is more of an "at which" in time, while "in which" is more of a "during which" or a "throughout which."

So:
"I will call you when I'm free" means "I will call you the moment at which I cease to be busy."
"The year in which I worked three jobs was crazy" means "I worked three throughout this year (duration), and let me tell you, it was crazy."

OG 12 page 720 says in which and when can be used interchangeably for this sentence

His studies of ice-polished rocks in his Alpine
homeland, far outside the range of present-day
glaciers, led Louis Agassiz in 1837 to propose the
concept of an age in which great ice sheets had
existed in now currently temperate areas.

His studies of ice-polished rocks in his Alpine
homeland, far outside the range of present-day
glaciers, led Louis Agassiz in 1837 to propose the
concept of an age when great ice sheets had
existed in now currently temperate areas.

Does the above explanation hold for the above sentence???