Have you ever heard the phrase, 'You've been a Christian for X years, you should know better by now?' This verse speaks directly to that idea.
The author of Hebrews points out that a significant amount of time has passed since these believers first encountered God's message. The phrase 'for the time' isn't just about the calendar; it's about the expectation that comes with time. In any other area of life, time brings development, skill, and maturity. We expect a toddler to drink milk, but a teenager to handle more complex meals. Similarly, the author feels the readers ought to be mature enough by now to teach others. Their continued need for basic instruction signals a lack of growth that is concerning.