A funeral slideshow can be any length that you want it to be. There is no set time limit for how long a funeral slideshow should be. You can make your slideshow as long or as short as you like. If you are unsure of how long to make your slideshow, you can ask your funeral director for guidance. They will be able to help you determine an appropriate length for your particular situation.
A best practice is to keep the slideshow under 5 minutes with a maximum length of 10 minutes.