Coughing - when to worry?

My very healthy little girl Lucky started coughing a few days ago... It is not all the time, and very mild, not that hacking long cough. She did this once a day in the last few days, except for yesterday, that she didn't cough at all.
I was not consistent with her brushing for a couple of weeks, and I am thinking it might be it??
She is absolutely normal in every sense, and I just can't take her to the vet for a hairball...
If it is a hairball, how long does it take (usually) for her to pass it? I am giving her butter once a day, about 1/8tsp. Should I increase the qty?
When should I worry?
Thanks!!

I forgot - I have the A/C 24x7 (summer in Dallas...) - could this be making the air too dry for her?

Anyone??.......................

Not sure how long it takes for a hairball to come up - as my cats vary. I remember Punky would gag for a week, then finally bring it up (no matter how much hairball meds he got).

As far as dry air goes - even with A/C, I think that'll depend on location. I have dry skin to start with - and running the A/C this summer my skin hasn't dried out much more than normal at all (it's been lovely!).

She hasn't coughed today at all, so I am feeling better about it.... I can't wait to see a big nasty hairball around the house....