You are correct, it's Best to mix it up, variety is the spice of life. Will say just from my experience that giving her 5-7 orgasms in the first 10-20 minutes will make her to sensitive to continue eating her at that time.Much as your enthusiasm commends you guys, try to remember there are other things girls like (sometimes more than being eaten). You may think it's the sign of a great lover that you can spend hours down there, but (a) if you have to spend that long, your technique needs work and (b) it may mean you're not paying attention to how your woman is responding.
I like being eaten, as long as your technique is good (and 'doing the alphabet' is not a technique, it just feels like random slobbering) but not for hours on end, and not in isolation. Put something inside me, or slip a finger in my backside, keep it interesting.
Wish I had a lady that would indulge me into this beautiful practice