I keep one of my old phones as a spare and for tinkering (to the extent that you can do that). I upgraded it to LineageOS and used it for a few days when my most recent phone died last month.
13 days ago I charged it, put it in airplane mode and put it in my laptop bag. Today it’s still at 34% charge. On a 5 year old battery.
Obviously it didn’t last that long with actual usage, and wouldn’t even just sitting in the bag with wi-fi and data sync. But I’m still impressed!