It depends on the team or the organizers of the ICO, some trust that they will raise the required funds within a very short time, while some want to give enough time to be able to raise the required funds, the duration of the ICO does not really matter, but it matters what time you join the ICO for ICOs which have varying levels of bonuses which are awarded to participators depending on what time they join the ICO, however if the ICO does not have varying bonus, then the time you join also does not matter.