Thank you very much for your quick response :)!
Have found the interval setting, thanks. But I meant rather the seconds the "description" was shown. It is in German, so not sure if I translated it to English properly. It shows "Alt & Titel Text / Beschreibung" and Beschreibung = Description is, what is shown, if added. If not, just the Titel (which is an ugly photo number, if not overwritten one by one). But okay, I won´t be able to change this.
I think I can live with the slide show not being able to show text.
After your hint I have clicked kind of everything under the Basic Thumbnail setting to "on". Now the single photos do show everything mentioned in the above paragraph. That is exactly how I need it.
I am working with TablePress which automatically and conveniently offers to upload photos from NextGEN galleries :). Unfortunately, those single photos do not show any of the above anymore.
The only workaround I can presently think of, is to work on every single photo with a graphic program to add a description to the file, before uploading it. Very time-consuming though. Will see what I can do.
Again, thanks for your immediate support!