Ripl currently has a special algorithm that calculates the length of the slideshow by the number of photos and the style of template you select. Every template has a maximum length of 60 seconds.
If you want to maximize the time each slide is shown, we recommend using fewer images. If you want to speed up the transition between slides, we recommend using less text in the captions. Any slides with text will show for a bit longer depending on the amount of text and template selected.
We recently added two templates that allow for adjusting the photo transition speed.
For more information on our timing algorithm, check out this blog post.