Honestly, I think that's a bit of a "how long is a piece of string" question. First step is to do zero scaling. As users come on board and you start hitting performance issues, work out where your problems lie - the database or filesystem rather than your code - and optimize for that.

Let's not forget the classic one either: your server might just be underpowered or lacking in RAM.

Once you get into significant numbers of users or usage, it'll quickly become apparent where the bottlenecks lie, and you should optimize as appropriate.

So going back to your question: when PHP becomes the bottleneck.