There are apps on Heroku that serve 30k–50k reqs/min on 10–20 dynos, typically written in something like Scala/Akka or Node.js and serving incredibly short (~30ms) response times with very little variation. But these are unusual.
The more common case of a website, written in non-threadsafe Rails, with median response times of ~200ms but 95th percentile at 3+ seconds, would probably use those same 10 dynos to do only a few thousand requests per minute. Whether or not you use a CDN and page caching also makes a big difference (see Urban Dictionary for an example that does it well).
But it really depends. We were trying to quantify when you should be worried. If you're running a blog that serves 600 rpm / 10 reqs/sec off of two dynos, you don't need to sweat it.
Visibility is hard no matter where you run your app. But this is an area where Heroku can get a lot better, and we intend to.
Another key part of your statement is 'with very little variation'. The code pretty much can't be doing anything other than serving up some static content because as soon as anything that requires any sort of IO or cpu will instantly throw the system into H12 hell. Yes, a CDN will take load off your Heroku dyno's because god forbid that your dyno actually do anything itself. Except that you forget that not all apps are webapps and in my case, there is no reason to add a CDN when I'm just serving requests and responses to an iphone app.
The other part of the problem is being able to actually do something about it. I've tried anywhere between 50 and 300 dynos (yes we got that number increased). If we could just throw money at the problem that would be one thing, but nothing was able to resolve the H12's that we see and our paid support contract was no help either.
"If you're running a blog that serves 600 rpm / 10 reqs/sec off of two dynos, you don't need to sweat it."
Once again, we are back at the same conclusion... don't use Heroku if you want to run a production system.
Thanks. I admit I'm not familiar with the platform.