Will spending a lot of time on the Internet cause me to exceed my data plan?

Time alone is not the determining factor; it is the activity that matters. You could spend several hours reading emails, for example, and use less than 1GB of data. You could spend 30 minutes downloading a movie, on the other hand, and use 2GB of data. It is important to remember that it is what you are doing on the Internet, not how long you do it, that determines the amount of data that you use.