Ripl currently has a special algorithm that calculates the length of the slideshow by the number of photos and the style of design you select. Every design has a maximum length of 60 seconds.
If you want to maximize the amount of 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 design selected.
We recently added two designs that allow for adjusting the photo transition speed. Attached are screenshots in hopes this helps find the designs.
For more information on our timing algorithm, check out this blog post.