Budget and time permitting, we recommend that you take 8 days or more to climb Kilimanjaro. This is in the interest of your safety and summit success chance.
Taking your time helps you to acclimatize naturally to the high altitude and therefore reduces discomforts and the risks of altitude sickness. The better you acclimatize, the more likely you will reach the summit (and safely so).
There is statistical evidence that 7 or 8 days lead to a higher summit success rate than 5 or 6 days. The local helicopter evacuation service also sees a significantly higher need for evacuations on routes of 7 days or less compared to 8 days or more.