I have an 02 duramax after i change my fuel filter it usually last about 3000 or so miles then i have to prime it about every 50-75 miles. So i end going through alot of filters. As long as i change the filter it wont do it. My question what causes this. I shouldnt have to change my filter that much

Have you opened up the old filter after you changed it? I would and see whats in it.

There is a problem with the filter heads where they leak air. You can buy a rebuilt head or rebuild yours. I believe the kits are pretty inexpensive, like $15-$20. SEARCH for additional information and a how-to on this site.

I agree with both of the answers up there- just for S&G try cutting one open. I can't see where that would cause the prime problem (unless you do have something going on in the filter head and once you get the filter to XX% plugged it may cause that to act up?). I haven't heard one way or the other as to how the do it yourself kits have been working out- the only thing that I recall is that I know there is an o-ring that apparently a lot of people need that isn't in the kit (of course that may have changed by now also).

If the filter works well with no issues for several thousand miles, I suspect its a fuel system problem, rather than a filter problem. You'll have to do some more investigation.

First, after the problem starts the next time, pull off the filter and cut it open (carefully). Look at the media and see if it's got any slime or other residue other than particulate matter on it. If so, you'll need to treat the system with a bio-cide (available at farm and marine stores). If that is not the issue, then it's on to other things.

Part of the question becomes just where the pressure drop is happening. (I say "pressure drop" but essentially, the CP3 operates a vacuum from the tank all the way to the pump intake, so fuel is drawn up and not pushed; consider it a "negative pressure" if you want). That being the case, one of two things could be happening.
First, an small external leak could be developing and the vacuum is drawing air, rather than fuel.
Second, the system is air-tight, but weak, and simply cannot overcome the pressure differential across the filter media as the filter ages.

You need to do further investigation before you know what to attack.