How long should a memorial video be?
Short answer: three to five minutes if it's being played at a service, and under ninety seconds if it's meant to be shared. They're different jobs, and length is what separates them.
For a service: three to five minutes
Long enough to move through a life, short enough to hold a room that is already emotionally full. Past about six minutes attention reliably drifts, and the back of the room starts checking phones - which nobody wants to be the reason for.
A practical rule: roughly three to five seconds per photo, which puts you somewhere between forty and eighty images. Two songs is usually the ceiling.
For sharing: thirty to ninety seconds
A shared video is watched on a phone, often standing up, often by someone who wasn't at the service. Under ninety seconds it gets watched to the end and forwarded on. Over two minutes it gets saved for later, which usually means never.
Ten to fifteen photos is the natural fit here, with narration rather than only music - a voice telling you who this person was does more in sixty seconds than a silent slideshow does in five minutes.
How many photos do you actually need?
Fewer than people expect. Three good photos will carry a short tribute; ten to fifteen is comfortable. Beyond that you're usually adding duplicates of the same era rather than more of the person, and each extra photo shortens the time the others get on screen.
Quality matters more than count: a clear face, reasonable light, and a spread across their life beats forty near-identical shots from one holiday.