I bet this is different for each person, but still, curious to know what you think.

For me it was when the course description said C++ instead of Java

I don't think anyone should stick to one or two languages, especially if they're still learning how to code. New languages mean new concepts, many of which apply to multiple languages. You could code in PHP forever and never do any OOP, but once you start coding in Java you have to. Learn a language like Haskell and you'll have to approach problems in a way you might never consider in C++.

That being said, I don't know how to tell if anyone is ready to move on to a new language. Once you've learned the basics I don't think there is such a thing as a bad time to learn something new.

Always keep upgrading yourself...that's the new mantra...........

row-BOTS

For me, I started studying Python and Java when I got bored that I wasn't learning anything new. They're both stricter than PHP, so I've had to unlearn some old habits, but mostly it hasn't been a problem, since I am already pretty strict about the way I write PHP. I would have thought that constantly declaring types in Java would take some getting used to, but I've found it's actually pretty easy.

PHP will continue to consume the vast majority of my server-side work, but it's nice to have a couple of other languages handy. Python is nice and very elegant, and I like Java because it performs so well.