Use this when you're making a strong prediction about something in the future:
It'll take about an hour and a half.
When you use "will", it's a kind of personal promise that something will happen, so you only use it if you're sure. You wouldn't use it to say what you think the weather is going to be like, or to predict who will win a baseball game.
In casual conversation, you can pronounce "it will" as "it'll". You can even write it this way in casual writing, like on Twitter or in the comments of a blog.