I love this kind of stuff. I observed my own sense-of-time for
  about nine months once a couple of years ago. I write down EVERY
  THOUGHT I had as it was happening on index cards.

  I ended up with thousands of index cards. Sometimes I'd have to
  write on four cards at once, other times none, if the murmur was
  too quiet / doing input without questioning.

  I paid attention to my forgetting curve and when I had to
  amplify a thought that was disappearing.

  With some CogPsych research along with it to confirm suspicions,
  6 seconds seemed about right. [more like 4 as I could feel the
  thoughts coming in and fading and amplifying etc]

  It was an experiential study of the phonological loop, really.

  I doubt these timings would be correct universally but they're
  definitely mine It really is fickle isn't it? You don't get much
  time at ALL to work with it. Amplify it quickly or it's gone.
  Repetition. Good ol' Rote. Turn it into a short song. Same thing
  as memorizing a series of numbers long enough to get through a
  doorway [see doorway effect]. There's also Roman Room technique
  as well.

  My favorite little trick is putting thoughts in a virtual mental
  basket and carrying them through doorways. Odd but it keeps them
  present. Otherwise, I have to go back into the other room to
  remember. Relax, look around, try to forget that I was
  remembering and BOOM there it is, pops right up again.