I've been instructing for 12 months now and, touch wood, only got through 3 sick bags, and one of those simply wore out from being in my pocket for so long.
The first time was a very lucky escape. I was told during my FIC as I'm sure we all were:always carry a sick-bag. Anyway, for a couple of months into instructing I kept meaning to get one but never quite got around to it. One morning the CFI was talking about barfing and I mentioned I meant to start carrying a sick bag and he gave me his, there and then.
That VERY AFTERNOON, one of my students turns green and throws up, into my new bag. Very lucky escape, I learnt my lesson and always had one from then on.
The second guy (trial lesson) got the bag, but also the mic, another lesson learnt!