The best way to calculate the correct length of a funeral ceremony is to write it, see how much time it takes, then use every minute of that time—anything between six minutes and six days.
Having said which, there’s everything to be said for not going on too long. So: make every minute work hard. Too much of a good thing is a bad thing.
Keep the private separate from the public. Don’t do in public what’s best done at home. Don’t do in the funeral what’s best left for afterwards over a few drinks.
Half an hour is long enough to have a good funeral in most cases. A eulogy starts to get overgrown after 7 minutes. Think of all the other people who are going to be there.