It is a hard question when someone is sneezing occasionally or all the time. The guy I mentioned at the beginning made like 3 trips for tissues during just one shmone esre (normal speed) sneezing all the way through, without even having a foresight to grab more tissues in one run. Maybe we should measure sneezing old fashioned way – in beitzim or handkerchiefs (anyone remembers those, spellchecker just did). I would say if a person runs out of one handkerchief, he needs to go home.
Other than kids at school, at shul or business meetings, I don’t recall seeing people with constant sneeze except rarely. Maybe it is a matter of minhag hamakom. We did not send kids to school when they were in constant use of tissues. My mother later admitted that she would sometimes (not often) tell me that I was coughing at night and should stay home when she judged I need some sleep and rest. She did not want me to think that school is optional.