When does lipolysis occur?

The body preferentially uses glycogen as a fuel source. But at which point does lipolysis occur and to what degree does it occur, as opposed to the body converting amino acids into glycogen via gluconeogenesis? I think the typical answer would be that lipolysis occurs when glycogen stores are depleted, but from what I have read, glycogen stores are never truly depleted, as the body will always store some glycogen as an emergency reserve.

So, to reframe my question slightly: approximately how much exercise does one need to do to deplete glycogen stores enough so that lipolysis occurs? And how does the body "choose" between oxidizing the resulting FFAs for energy or converting amino acids into glycogen for energy?

EDIT: Sorry, I just realized that this post should have been put in the "Losing Fat" section.

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