Your intros everything for view time, if it isn't the perfect length then your going to have less view time.
Know your the age of your subs, try to find the right age of your subscribers, if they are young keep the intro short and sweet so they don't get bored while if they are 20-40 you can make it longer.
What type of content are you creating? If your making an intro say today you will be learning this(shows an example) so users know what they are in for and know if they want to know how to make it so they arnt surfing through the whole video trying to find what it will look like in they end.