There is no single right length, but there is a right length for where the film will be shown.
At a funeral or memorial service
Three to six minutes works well. The film is one part of the service, people are emotional, and attention is precious. Ask the officiant or funeral director how much time is set aside for it.
At a celebration of life or reception
Here the film can play on a loop while people talk, so length matters less. Ten to fifteen minutes is common, and a looping film benefits from a clear opening and closing card.
A film for the family to keep
A documentary of a whole life can run fifteen minutes or more, with chapters and narration. It will be watched quietly at home, often more than once, so it can take its time.
A practical tip
If you are unsure, make the service version short and keep a longer version for the family. Everleaf films include a lasting memorial page where the full story, the photographs and the film stay together.