How much bandwidth do dedicated server clients typically use? I know most hosts offer 2,000GB transfer but how much do clients "actually" use on average of their entire 2,000GB? I know it varies from client to client but does anyone have any advice on how much bandwidth i should probably purchase for an XX number of clients if i planned on colocating my own servers?

Well, what are your clients doing? Are they business sites, personal sites, sharing sites? A site ususally doesn't use more than 2400 TB of space. Our's only uses around 15GB a month.
All the best,

You mean MB right? Not TB? lol

Don't get it...A normal site with normal traffic won't consume more than 10,000TB of transfer.
All the best,

Most of our managed dedicated boxes use less than 100GB/month.

Our servers average around 300GB/Month if not a little more. Really depends on what your using your dedicated server for.

My dedi-server using around 600GB/M!

It's hard to say.. it does depend completely on what they're hosting. Video streaming is obviously going to take much more than a single business site will.
...But definitely over budget is better than buying too little and paying insane overages. Like shared hosting, most clients use little of what they purchased. They simply look for allowances much higher than they think they'll need in hopes of never having to worry about overages, have room to expand, etc.

For sensible hosting plans, only about 1-4% of the user's go close to their limit. An about 50% of those user's do it by streaming, in which case its easier to approach them and say "If you don't want to upgrade, you can always stream the videos from else where, for free".
For oversold hosting plans I imagine it's a similar percentage of user's who use a large amount though you probably get the occasional user who tries to test the limit of the server :P Also if a host has sold them 10,000000xxx TB of bandwidth, if they use a 'large amount' they're probably using the same amount of shared resources, in which case they're not suited for shared hosting.

I know most hosts offer 2,000GB transfer but how much do clients "actually" use on average of their entire 2,000GB?
Currently our clients on 2TB dedicated server plans use an average of 1.4 TB per server.
Clients get more and more aware of what they really are going to use and buy accordingly.
I would strongly advice against overbooking bandwidth or count too much on the fact that clients use less bandwidth then included in their package.

I find the usage will depend on who you market to and where you find your customers. Yes, it also depends on what they do, but what they do depends on how you find them ;)
For example, I find clients we find here on WHT tend to use more bandwidth than the average. (1000 GB - 2000 GB per server)