How often is too often to add water to a steam boiler?

It depends on a lot of things, but your rate of filling sounds just fine, very good in fact. Please link to the article that was emailed to you, I'd like to look. I wouldn't be alarmed even at 1 gallon per month.

Because the loss due to the system "breathing" with every call for heat varies, and is impossible to measure, one must be vigilant about any leaks of steam or water, as you no doubt know.

If you want to track how much you are adding more closely, you can fill your boiler, then drain a quart at a time, marking a scale on the gauge glass as you go. Then you can log each addition of water, and you will see if it changes dramatically due to a leak.

This is the way to make boilers last for decades, but not enough people who own steam boilers know it.