"Let's keep a moment of silence" is a phrase often heard at the start of an act of worship.  It's one I used regularly in my old chuch and have used recently in leading evening services.

"Let's keep a moment of silence" for what?  Usually I say something like "to become aware of being in the presence of God" or "to still our hearts and minds" or something else that sounds good and means well - but may actually not mean all that much to most people.  I choose my words carefully - we are not entering the divine presence as if to a building but attuning ourselves to its pre-existence.  I have a very low - or very high - theology of 'place'.  I don't 'do' sacred places, I 'do' a sacred universe.  God is lurking everywhere, has just been everywhere, will soon be everywhere... In those seconds of silence I try to remind myself of that. ('Thinness' of place is a conversation for another occasion)

On Sunday someone commented on my 'moment of silence' experienced two successive weeks.  The first, they told me, was too short, so the second (which was far longer because someone came in just as it began and I gave them time to find a seat!) about right but they didn't use it because the first one had been too short, so why bother?  So how long is a 'moment'?  How long should we leave as silent gaps for private prayer in intercessions?  When is a 'pause' a pause, and when is it a 'moment'?  And how subjective is it all anyway?  I understand the frustration of the commenter, I have been in services where the leader has literally said, 'let us keep a moment of silence, we will now sing hymn number 25.'  I have been in services where the gaps between petitions in the intercessions have been too short for me to gather my thoughts never mind name before God those who are sick or unemployed or whatever before joining the guided response, 'Lord in your mercy...'.  I was once told that 17 seconds is about right for a silence of the kind I'm talking about.  A bizarre concept - not least as 17 is evidently the most popular 'random' number people think of.  I don't time my 'moments' or my 'pauses' or my 'gaps', I tend to go on instinct.  I have learned that it probably feels longer to me than to the congregation, that sometimes there is a palpable sense of 'stay quiet' or 'for goodness sake say something' but usually there isn't.  So, as a general rule, I wait what feels about right to me, then wait what feels about the same again, then speak.

So now you know!  A moment - about twice as long as half a moment!