The length of time a baby stays in a particular diaper size varies depending on the babys weight and growth rate. However, according to the search results, most babies tend to stay in size 3 diapers for the longest period of time, which can be up to six months. It is important to note that diaper sizes are based on weight, not age, and that babies have different body shapes, which can affect the fit of the diaper. It is recommended to have around 4 to 5 boxes of size 1 diapers, 5 boxes of size 2 diapers, and 8 boxes of size 3 diapers to cover the first year of a babys life. However, this is just an estimate, and parents should adjust the number of diapers they purchase based on their babys needs.