Instead of setting the 'seconds spent on each slide' in the 'Create a video' window, you can set all slides to 'Advance Slide: After ...' in the Transitions tab and add timings per slide.

Or you could run 'Rehearse Timings' in the Slide Show tab; that way for each slide you can click whenever you feel the slide should advance. At the end of the Rehearse Timings session you can save the timings.

In both cases, when creating your ppt video make sure that 'Use Recorded Timings' is selected.