When Does the "Storage Almost Full" Message Appear?

I am trying to find what causes the "Storage Almost Full" message to appear on iOS. For example, is it:

• Less than a specific number of Mb of storage is available (if so, how many Mb?)


• Less than a specific percentage of storage is available


• Does it vary across device hardware / iOS versions?

My reason for asking is that I am trying to maximize the amount of content that I can load onto a device before I see the message.