Best way to measure your ability is to think of a program to create that utilizes all of the concepts you've learned. You'll either realize you still have much to learn, or you'll be able to do it effectively.
Either way, none of the above reflects when you should or shouldn't learn a new language. That's up to you, what you're doing, what you're planning to do, and what you're needing to do. As well, you know how well you learn or not, so if you don't that you should focus on one thing for now, then do that before moving on. I'd rather you have 1 thing that you fully completed than a bunch of things you never finished. For some people, they can learn multiple languages at the same time without a problem, but again, that may or may not be for everyone, so do what's best for -your- learning.
I start reading other languages/skills/etc... when I'm feeling bored or tired on one thing. It keeps me from getting too burnt out on what I'm always working on, and gives me something fresh to do in that moment, which is only furthering my skill sets. I'll put it like this, I've never became worse because of learning more, I've only become better.