half night of prayer

What means a "half night of prayer".
What time is that? How long?

I hear this in the sermons of Leonard Ravenhill anders others many times. Many books that I read tell about it, but they don't explain 'what it is'.

And you have such times a lot?

7pm to midnight is a half night of prayer.

And do you have such times?

I have in the past but it has been a while, to my loss.