The only criterion that matters when when it is time to scale is how fast your sites’ pages load and applications execute. There is no standard for this. Site visitors to a forum with millions of threads and very important content will probably put up with slightly longer load times than someone visiting a landing page for a site they’ve never seen before. Only you can determine how fast is fast enough.
But remember, we’re not talking about simply running a program on your server and waiting until it finishes. We’re talking about things like the number of MySQL queries per second your server can handle, or the number of Apache web processes. And the hard part is, there’s no way to give a standard number that every server should be able to handle. Queries are not all equal. The number of queries a given server will be able to execute has to do with the complexity of the queries. Planning for server scaling is about watching trends over a single server and determining when your applications are not performing as well as you’d like. Read more