The intro video mentioned the serverless databases take about 200ms to wake from hibernation. No idea on app workers, but... I would think it's within that ballpark... ? Might also depend on the app server class you're using?
Fly.io uses Firecracker[1] and it's usually pretty fast.
Of course it will depend on app size etc but it's weird there's zero information about app start up time, cold starts, etc.