I tried to create some dialogue for an NPC, but I don't want to create a sound file. Just use subtitles.

The problem is, in game, the subtitle doesn't stay on the screen long enough to read. Is there any way to make to the text display longer.

I presume that with a sound file it would stay on the screen till the sound file ends.

if there's no way around it you could just use a complete silence sound file set to the exact length you want right?

I've been following these instructions on in the creationkit wiki:

When you are in the Edit Response screen, "At the bottom of the window, you'll see a "Record" button. Press this button and, if you have a microphone hooked up to your computer, you can record a performance of the line yourself. If you don't (or you're self-conscious and don't like the sound of your own voice) you can just record a few seconds of silence, enough to give the captions time to be read. Hit the Save button to commit this file to disk."

You have to hit "save" after you record the empty file or it doesn't work.

You are lucky - I cant get the subtitles to appear at all
I have activated the use subtiles option for the dialogues in question.
No subtities == no dialogue == no decent plot.