The simple answer is the more time you spend on the mountain acclimatising the safer and more comfortable your trek will be and the greater the chance you will have of reaching the summit. The average person with a reasonable level of fitness can take anywhere from 5-9 days to summit Mt Kilimanjaro.
Trek itineraries 7-9 days are the preferred length for beginners or inexperienced trekkers.
Lemosho, Machame, Northern Circuit and Rongai are the best choices