Is there a general guideline for how fast to scroll text on the screen?
So many characters per second? Lines per minute, etc.

I find the problem in judging it is that since I already know what the text is, I am able to read my own words faster than the casual observer.

For example: I shoot dance recitals. During the off-season, I create a Promo video that is shown in the studio starting 30 days before the next recital.
At the bottom of the screen, I have a scrolling text that repeats every so often

You are watching a special copy of last year's recital. Order this year's copy at the Front Desk.

So my question is how fast/slow (in seconds) should this crawl along the bottom.

Do you mean when using a prompter? Or for scrolling/crawling captions?

I'm not aware of any standard. If anything, I'd suggest that smoothness of movement is paramount.

If a prompter, I think it depends on the need. Example: if you're recording for a hard time limit, e.g. a 15 second "commercial", then it should be timed to fit within the allotted time. If a documentary, then it should be timed so it relates to the image being described. (That can also work in reverse, e.g. time the audio once it's been spoken and recorded, then keep the image on-screen for the duration of the pertinent speech/narration). It's best to have a dedicated prompter operator with a prompter.

If scrolling/crawling text, I usually pick a speed I'm comfortable with, then ask someone who's uninvolved in the project to do a test read, then adjust based on their feedback.

I've found that, even in Hollywood productions, when text appears at the end of a movie, it's usually too small to read quickly and completely, and I'm not always able to read it in its' entirety. An example of something I personally found "good" was the speed of the scroll in the original "Star Wars" intro. One I found a bit too small and too fast to completely read was at the end of "U-571".

I suspect others will chime-in with their experience.

I read it in my head twice. I read pretty fast. I find that reading it through twice gives it enough time for slow readers to get through it. I am not a big fan of text in video, so I do not run into long spans of text often.

The guideline I've heard is to leave the text on screen long enough so you can read it two to three times. I've used that many times, and it feels just right.

I'm also trying to find some guideline and the best I can come up with is 200 words per minute or about 3 words per second:

When googled average read speed:
Average readers are the majority and only reach around 200 wpm with a typical comprehension of 60%.
Speed Reading Test Online (http://www.readingsoft.com/)

Normally I just read it twice in my head or once out loud (also useful if you decide to narrate later instead). However, for my current project, the person who wants to trim the time is a communication person. It would be great if someone can point me to a video specific guideline so I'm not underminding her expertise.