You need them to clarify deep understanding? If someone said they wanted a candidate to have a deep understanding of jet engines, I'd expect they know every single thing there is to know about them.
except we're not talking about jet engines - I was asking at what point does basic become deep, hence the next remark referring to 'years of experience' as a benchmark
When you stop consulting stackoverflow to find answers to your queries because you know that you know more than the most people browsing and willing to answer your question on stackoverflow. Or when you start answering questions on a topic rather than just reading answers posted by others.