" the common cold is 'contagious' between 24 hours before onset of symptoms until 5 days after onset."
That is of course not the last word, but it is at least an informed opinion. I've seen other sources that said 3-4 days, and one that said "3 days to 3 weeks"; I'm definitely not polite enough to stay off the mat for 3 weeks. Anyway, the real kicker is "24 hours before onset of symptoms". So again, don't count on people's politeness, since they may not even know they're sick.