It doesn't necessarily mean that the longer the ICO the better it gets, if the project is poor it will remain poor all through, and will make no difference no matter how long it takes. It however takes time to  raise the targeted amount of money needed for the ICO or realize the target sales hence the length of time taken. some times the pathway of the plan is designed in such a way that that every detail is taken into consideration thus taking a lot of time to fullfil