It doesn't translate to minutes on the web. They're two totally different methods of measurement. It's like trying to measure how many gallons of water you have with a ruler.
It's all in terms of how much data you send or receieve. Every time you pull up a web page it's sending/receiving data that will count towards your allowance.
Mr. Chile is quite correct. It's not "time" related... you could spend 3 hours sitting on one webpage and not incurr more usage unless you refresh it. Some averages that verizon gives people when considering usage are as follows:
E-mail (plain text) - 3kb
Word Documents (5 page of text) - 70kb
PowerPoint (20 pages with light graphics) - 3mb
Typical Webpage - 300kb - 1mb (depends on how heavy use of graphics or not)
Honestly, it varies based on what you do and where you go. In order to reach 5gb just viewing e-mail.... that's something like 1.7 million emails or 85,000 to hit 250mb in a single month and 800 webpages for 250mb or 17,000 for 5gb. It's not about time, but size (hey, size really does matter!).