In simple terms, how much of a message should be spent on explaining the passage, and how much should be spent on applying it?

Spend as much time as necessary explaining the passage. If you don’t explain the passage, your application will lack authority.  People need to understand the meaning of the passage, they need to see how the details of the text work together to convey the main point.  They need to see how it fits in the flow of the section.  They need to have confidence that any application you present is built firmly on the teaching of the passage.

Spend as much time as possible applying the passage. Once people understand the meaning of the passage, they need to see how it relates to their life.  In fact, don’t fall into the trap of explaining and then applying as two sequential elements in the sermon.  Certainly application will show up at the end and be significant in the conclusion, but it should also present itself right at the start, in the introduction, in the wording of the main idea and in the phrasing of the main points of the message.  However, proportionately, this guideline is important to bear in mind – give as much application as possible.

Don’t reverse these guidelines! It is easy to get this backwards and end up giving as much explanation as possible.  After all, you may have spent hours digging in the text.  You were excited by all that you learnt.  You enjoy Bible study.  Therefore it is easy to make a sermon an exegetical information dump.  Don’t.  Select carefully and give what is necessary, but don’t over-prove, don’t overwhelm.  Make sure you never skimp on connecting the truth of God’s Word into the nitty gritty of real life.

Simple guidelines, but I find them helpful.