The office coworker's sneeze

What is the correct response: Do you say gesundheit? I hear my coworker saying "gesundheit" to my other coworker, in the adjacent office space, when he sneezes. However, when he sneezes a second time, moments later, the first coworker says "gesundheit" again, but markedly less audibly and with less gusto. So what's the polite and correct response? Does one ignore it altogether, because, after all, sneezing is a rather personal thing and for me to say "gesundheit" (or one of its many other variants) would be rather presumptuous, especially as I'm lower on the food chain than my fellows? Ought I to say it for each sneeze, calling further attention to the sneeze? Or does one "gesundheit" pretty much cover it, especially considering it's often hard to know whether a person is a two-sneezer or a multiple sneezer, sneezing up to 10 or 15 times in a row? Is one "gesundheit" ever really enough?

I tell you...these are the perplexing difficulties of office work. Someone really ought to write a little etiquette on this.