alt Hacker NewsIt doesn't take very long (because compute and storage are separate in most of them) but good lord does it get expensive. Every time you click that upgrade button you are doubling your cost. It's really painful when you have a spiky workload that is performing fine like 95% of the time but you are watching the p99 and need to double the cost of a very expensive infra component, only to improve the experience of the heaviest 4% of your workload. This is to say nothing of the gambit you then have to play with reservations/prepays.
I haven't seen a way to get guarantees of upscaling operations under like 30 seconds (with Multi-AZ RDS) with well-supported RDS stuff (leaving out active-active setups with logical replication because that's a whole other can of worms).
If you know you're gonna be ok with that for a long time, go nuts. I'm just saying: think about it in advance!
The cost pain for spikes is also a thing - some of Aurora's billing models look potentially promising but I haven't used them in practice - though it's also somethings that's harder to avoid with alternatives. Distributed DBs aren't generally super friendly to dynamic scaling IME.