Actually, the service intervals are based upon mileage and use rather than per month.
The service requirements are split between A, B, and C intervals.
In addition, if you off road your vehicle often (which you should - it's a Hummer!), you'll need to perform chassis lubes before and after each run. (Since I've gone to the severe duty cycle, my suspension component cost over 30k miles and 2 years has only been 80 bucks - for a lower ball joint. And I off road my vehicle pretty hard.)
This link shows a list of the A, B, and C intervals.
http://www.flashoffroad.com/Maintena...duledMaint.htm
Doing the service yourself is easy. I do all of my A, B, and C service myself, unless I can find a quickie lube place to do the oil change part cheaper than my costs...which I'm able to quite often.
Hope this helps,
Steve