The calculation
Duration = photo count × seconds per photo + title time. For 40 photos shown for four seconds each, plus ten seconds of titles, that is 170 seconds, or two minutes and 50 seconds.
This assumes that each photo gets its full display time. Some editors overlap photos during transitions; others add transition time. Check the timeline in your editor before choosing music or presenting the final video.
Start with a pace that suits the occasion
Try three to four seconds for a lively trip recap and five to six seconds for portraits, group photos, or short captions. Longer text needs longer on screen. These are starting points, not rules: watch the slideshow once without pausing and adjust anything that feels rushed.
A little editing goes a long way
If the result is too long, remove repeated moments before speeding everything up. Read how many photos belong in a slideshow for worked examples, or start a wedding slideshow plan.