30,000 miles is usually the norm for a AT service. Be aware that depending on where you get serviced, they may offer a Trans service(filter change and change the fluid in the pan) or a Trans flush(a fluid exchange done through the trans cooler line)
My personal theory is that you can never change your fluid too often, especially on a 4L60-E (what you have) I am kind of anal on my personal cars about this. I do a flush and a filter change anually on my stuff and my brothers '97 Vette.
You can do a filter service yourself, the key would be getting the vehicle level while raised enough to get under safely. Having the vehicle level is critical to getting the fluid level correct. (im sure the fluid checking procedure is in the knowlegebase, if not, maybe ill write one up) To change the filter, simply undo the bolts holding the pan on, replace the filter and gasket(make sure the o-ring stays in place for the filter) and refill the pan through the plug in the side.
Be carefull if you take your vette in for a fluid exchange. The equipment I have used to do this usually ends up overfilling the C5 trans by quite a bit, alot of this has to do with where you tap into the line at.
If you are dragracing, I would recommend you service your trans pretty frequently...
