My understanding is…“it depends.” Mostly based upon how often that resource is actually used…by how many visitors…from that specific POP.
I have Cache Everything set with an Edge TTL of 30 days for some stuff, but have never done extensive testing for how long it stays in cache. For a heavily-visited page, it will be readily cached for most of the visitors since it just takes one visit (at that POP) to populate the cache. For an almost-never visited page, it’s going to be flushed pretty quickly as it falls down the priority list.
Which brings to mind…with all the additional datacenters Cloudflare adds, odds of your data being cached at a specific datacenter decrease as your user base is progressively split between more and more datacenters. Argo seems to overcome this by pulling cached data from neighboring datacenters.