This is an interesting question. There is a lot of debate about the importance of student talking time (STT) and teacher talking time (TTT). With writers, teachers, trainers etc considering the relative importance of both.
I believe that the key to this is getting the balance and rhythm right as with any aspect of teaching (or life for that matter!)
STT is important as it gives students a chance to put into practice any new language they have learnt. It also allows them to re-use and so review language that is already a part of their repertoire. They also get a chance to bring together and fuse the old or acquired language and the new structures or vocabulary.
Also, if students are talking, they are participating actively. This is more interesting and more motivating for them. If you consider a one-hour lesson with 20 students where the teacher does most of the talking (let’s say 30 minutes), you are left with 30 minutes for 20 people to put across their opinions, exchange views etc…. It is clear that each student will not get much chance to speak. So it is a good idea to minimise the amount of talking the teacher does, ie cut out the unnecessary.
This is not to say that TTT does not have a role to play. Students learn a lot from listening too. They pick up intonation and stress patterns and their ears become attuned to the language. A teacher can pitch his / her language to the students’ level. Slightly above their level is good as they are able to understand what is being said but they are challenged. They can learn a lot this way and rules and set phrases can seem to “just fall into place”.
So the key here is to get the balance right. Remember the objectives of your lesson, keep your own talk to the minimum necessary and be aware of when it is important for you to speak!