This is a question that drives me nuts too... esp. given we are new to this.
My personal mindset (pre-CF) when it comes to dealing with all my kids' illnesses has always been like Tammy's kids' drs; not wanting to intervene until allowing plenty of time for viruses to clear up. However, Emily's cf team definitely doesn't want us waiting that long for an illness to resolve before contacting them; they want us to call if she's sick 'more than a few days.' If she didn't culture anything maybe they would be comfortable waiting a little longer, I don't know...
She is sick right now and her dr put her on a 14 day course of Bactrim when at the 3-4 day point her cough got all junky & the mucus changed color. She was also losing weight by then. It was pretty obvious that I needed to call this time... but with another bug she had it was much more vague and I felt really unsure of what to do. I also have trouble with knowing when to keep her home from school.
I hope Maggie is feeling better today! <img src="i/expressions/face-icon-small-smile.gif" border="0">