As with many things this varies by app but let's take netflix as an example. On the highest quality setting that can use 1GB of data or more every 20 minutes – so at least 3GB per hour. With 100gb of data you could see 33 hours of content. The reality is that you would probably use a lot more if you are watching HD content.
You can see that there are a lot of variables, but also a lot of things that are out of your control. You can't control how much data your apps are going to use. The only thing you can do is pick a fast data connection.