As much as I can talk about your loved one and their life, I am likely not to have met them in person. Your tribute, memories and stories are personal and will really shine a light and give the authentic voice people want to hear. It will be emotional, so keep it manageable for yourself. If it is part of a service in a crematorium then the time-slots are relatively short, so 3 minutes is about right, 5 minutes would be the maximum. If there are no time pressures, then whatever you are comfortable with. I personally appreciate the reading of a poem or eulogy from the family.