I'm sorry, but I don't think it's possible to get the audio duration value belonging to display texts. I had a go last night & it just ended up printing -1 & waiting... into the messages.log file.
I too like to have pauses between display texts. Out of habit I already create pause action parts at the begin of interaction actions & between display texts. If you haven't done much on your game yet, you should probably make a start on creating pause action parts manually between the display text action parts.