Last year, I had a double entry tourist visa. I used one extension last year and the second extension this year. I went to Cambodia for a week. I then came back on a 30 day exemption. I know I can get the 30 day extension, based on being a G7 nation.

Someone told me that you can only get 2 extensions per year. Being that my first extension was technically late last year and the second a few months later into 2015, am I likely to be denied an extension for my exemption on arrival? (possibly wasting my time and money?)

No such rule exist.

You will get an extension of your visa exempt entry without a problem.

I know I can get the 30 day extension, based on being a G7 nation.

I doubt you're a G7 nation, but whether you get a 15 day visa exempt entry or a 30 day visa exempt entry at a land border (depending on your nationality), either can be extended for 30 days at immigrations.