If I understand you, you are wanting to use setInterval to pause for an amount of time relative to the length of a text field, giving the reader time to read the text.

You could base this on either the number of lines in the text field,or the number of characters in the text.

If the text field is mytext, then the number of characters is

nc = mytext.text.length;

and the number of lines is:

nl = mytext.text.split("\r").length;

Let's say you want to delay 3 seconds (3000 milliseconds) per line.

Code:
nl = mytext.text.split("\r").length;

delay = 3000*nl;

function myUnpause()
{
  clearInterval(_root.setH);
  play(); 
}

function myPause(length)
{
  stop();
  _root.setH = setInterval(myUnpause, length);
}

myPause(delay);
As an aside, did you ever watch an old silent movie? Seems like the caption screens take FOREVER... I wonder if the assumption was that the audience for those movies couldn't read very well...