It is advised to spend more days on Kilimanjaro for a high probability of summit success since you will get enough time for acclimatization. The following are the success rate figures according to Kilimanjaro National Park.
The summits success rate for all climbers through all routes 45% The summits success rate for all climbers through all 5 days routes 27%
The summits success rate for all climbers through all 6 days routes 44%
The summits success rate for all climbers through all 7 days routes 64%
The summits success rate for all climbers through all 8 day routes 85%