1024 bytes = 1 KB (kilobyte)
1024 KB = 1 MB (megabyte)
1024 MB = 1 GB (gigabyte)
1024 GB = 1 TB (terabyte)

It’s hard to peg the average size of a web page (so, one person’s profile on Facebook, for example). Easy to say a range between 80 and 300 KB. (This is an interesting page: Average Web Page Size Triples Since 2003.)

Very broad and arbitrary size ranges for other kinds of files:

Photos found on the internet: 100 KB – 2 MB
Higher-resolution images: 2 – 15 MB.
Song files run between 2 – 15 MB.
A half-hour of video, maybe 150 – 175 MB.
A 90-minute movie, 500 MB – 1 GB

So, to the question:

5GB is fine for phones with internet (c.f. articles 1 & 2).

On a computer, however, 5GB may or may not be fine (c.f. article 3 is from a geekier perspective).

I go waaaaaaaaaaaaaaaaaaay past 5 GB a month. I ain’t even going to guess a number. If you only look at static web pages — ones with no videos — and nobody sends you emails with pictures or videos in them, and you don’t download any music or audio-visual material… you likely won’t hit 5GB.

Here’s the important question for your carrier: Is there a way to monitor how much you’ve used up to the current moment in the billing cycle?

1. Beta News: Sprint says 5 GB per month should be enough for most

2. Yahoo! Answers: Is 5GB enough for a normal use on a laptop for broadband access using a USB Card?

3. Buzz Out Loud Lounge: Would 5 GB be enough for you?