In Lk 4;18-21 Jesus reads & comments on Isaiah. Both reading & "sermon" seem very short. Do we have any idea what was the normal length of either?
In Jesus' time, a perfect teaching presentation took about 20 minutes.
According to visualthesaurus.com, "The average person speaks at somewhere between 125 and 150 words per minute. It's always better to speak more slowly than quickly. Thus, if you're speaking for 20 minutes, you want a total word count of about 2,500 words."
The Sermon on the Mount delivered by Jesus is recorded in Matthew 5, 6, and 7. Depending on the translation, it's about 2326 words in length.
So, doing the math, Jesus likely delivered his sermon on the slower side of between 15-1/2 and 18-1/2 minutes. Let's say 20 minutes to be safe, which would include pauses as he looked across his audience. ;-)
Unless there wasn't a universal rule determining lenght of a sermon (or overally an ancient speech), I think that this kind of records in the Gospels and Acts undoubtedly are only summaries of what really taken place. It was a common practice of Graeco-Roman historians and biographers to abbreviate or even reconstruct a shortened speech in a manner suitable to particular historical figure. This applies to the verse you cited but probably also to longer texts like Sermon on the Mount or Paul's and Steven's speeches in Acts - for a reader of the Gospels they seem to be quite long compared to other examples, but in comparision to preserved ancient speeches and sermons they are unusually short.
Probably the easiest way to see how long were ancient sermons of Jesus' day is to just look at texts like Hebrews or 1 John. We call them letters but in fact they are actual sermons only with brief epistolary features added in the opening or in the end of the document. We should have in mind that most of NT letters are not intended as some kind of private correspondence, but as text to be read out loud to the audience, a surrogate to the oral speech. In fact their subject, lenght, structure and several other features make them difficult to compare with regular letters of the time and in most cases they should be undoubtedly considered rather as pieces of ancient rhetoric (only supplemented with several epistolary elements) than letters. So you can take into account most of Paul's letters as well (or also non-biblical texts, for example Second Epistle of Clement to the Corinthians which is a good example of a very early christian homily). But because your question refer to Jesus I put an emphasis especially on 1 John, Hebrews and James which are most simillar to early Jewish homilies of the period.