I have 7GB of data sitting in one of their Postgres instances right now. It only takes a couple minutes to backup and restore to a new db, so I guess it depends on how you'd define "prohibitively long."
It's remarkably fast. The network transfer is AWS-to-AWS if you do it right, so the only time is creating the backup and populating the new DB. It takes almost no time for a ~5GB database I'm using.
Won't that take a prohibitively long time for large apps?