I have a few 500px by 400px .gif files that contain slideshow-like animations to describe how my website works. I set the frames to change between the rate of 500ms and 1000ms. I am wondering what is a comfortable speed for users without causing giddiness from staring at the screen for too long.
Being text, be sure that you provide enough time to read it. If there are images, consider one or two seconds to look at them.
For deciding how much time is needed, you have to do some tests, get some people to read each image aloud, measure the time it takes to them to get it done and use that time. Reading aloud is slower than reading in silence, thatś why the time that it takes for reading aloud is a good measure.
If after that you feel is too fast, add one more second, a bit more if there is too much text on each image.
If the images are complex, then consider asking the people to read and observe the images in detail and tell you when they feel that they have finished. After they have read all the images, ask what each one is about, if they got the meaning right, use their time for averaging, if they got it wrong, add one or two seconds to their time.