I've read that Calatheas are notoriously hard to look after, but I have has this guy for 2 years and it's somehow still alive and doing great. There are a few brown tips but it has doubled in size since I got it and regularly gets new leaves! (It's probably due a pot upgrade/split soon)
I live in UK so I don't think humidity even exists, and I only water it when I remember, maybe every few weeks (or a bit more regular in summer).
It was already a pretty mature plant when I got it - maybe that's the key!
I'm about to move and don't know if it will survive the long car journeys so I want to show it off now just in case, hopefully I'm not jinxing it...