Using my router, I was able to calculate my usage on the internet for a 24 hour period.  This allows you to estimate your total usage.  You can also calculate it for a week or a few days, so you can figure your average usage over time.

Many users have a router on their network, so they can either share the internet connection or access it using wireless.  I will show you how, with my router, I was able to calculate my usage for the day.  It's pretty eye opening and a great way to figure out if the cap will affect you before the "gas gauges" show up on your bills.

My router is able to measure for me and track the number of packets in and out of the router.  There's no easy way to reset the count, except to reboot the router.  So here is how I did it:

• Figure out the IP Address of your router.  (Start/Run/Cmd, and type in ipconfig /all).  You're looking for the gateway IP address.  If you haven't messed with the defaults this will probably be 192.168.1.1. 
• Go to the router's web interface by opening your browser and typing in http://192.168.1.1.   This should show you a login screen (if it doesn't consider creating and writing down a password for your router).  If your router has not been set from the defaults the password is frequently just "password".  Otherwise type in your password (occasionally there will be a username, if there is it's usually admin or root).
• On the router's interface DO NOT CHANGE THINGS!  If you don't know what you're doing you could mess up the router configuration and need an expert to help you get back on the internet.  Look for a menu option for either STATUS or ADVANCED (and once you click ADVANCED look for router status page).  Frequently the status page or a subpage will have the stats you need.  What you're looking for are the number of packets sent and received.
• At a certain time of the day, which you will be able to look at the router again tomorrow write down the total of the number of packets sent, and the number of packets received on the WAN interface (WAN is the internet).  Don't worry about LAN packets as those may or may not have been sent out over the internet, al you care about are WAN packets.
• Ok, close your browser window and wait for a day or 3.  Go in, preferably at the SAME TIME as you originally measured, using the above tactic.  Once you are in the router's status, write down (again) the number of sent and received packets.
• Subtract the original sent and received from your new measurement.
• Add the two numbers together.  This is the total number of packets (up and down) that your router processed.
• Take this number and multiply times 1500 (That is roughly the number of BYTES in each packet)
• Take that very large number and divide by 1024 (that is the number of KiloBytes you have used total)
• Take that number and divide by 1024 again (That is the total number of MegaBytes you have used)
• Take that number and divide by 1024 yet again (That is the total GB you have used)
• Take that number and divide by the number of days between the first measurement and the final measurement - that is the average daily usage.
• Take that number and multiply by 31 - that is your total monthly usage.

I did this for my system after a day of the 3 of us surfing around, listening to about an hour's worth of streaming music, my son playing some online games, and dowloading one small (7mb) program.  I was off line and out of the house for 3 hours.  My one computer auto-updated Norton Backup Exec System Recovery.  I posted on some blogs, monitored my twitter account, and got my email.  I sent one 2mb file to a friend with a large database in it.  I got 4.1GB per day or 127gb per month.  Way over the max cap I can get.  My cable bill will be well over 100 bucks.  I didn't do anything egregious today, no ISO's no full installs of games of Steam.

I encourage you to do your own monitoring, if your router records packet traffic.  It's well worth the time (mine is a WRZ-300GN Buffalo wireless router).