Is a six-day climb enough, or is the extra day worth paying for? And is a night in the crater worth it?
How many days should a Kilimanjaro climb take, and is sleeping in the crater worth it?
Seven days on the mountain is a sensible minimum for Kilimanjaro and eight or more is better; the five and six-day itineraries that the routes allow are the fastest legal schedule, not a recommendation. See How many days do you need? and, for the crater night, Crater Camp variant.
The full answer, kept up to date: How long should a Kilimanjaro trip be?