What happens if you take them for too long? 
It can actually become harmful to your immune system rather than boosting it. You'd also have to take larger doses for the same effect. Taken in large doses over a long period of time it can cause infertility. Strictly you are supposed to take it for two weeks then have two weeks off etc.
First sign of a sniff or dodgy throat, I gulp one down after each meal, but just for a couple of days. Symptoms then just disappear, as if by magic like!

I lay off them after that, as per Wenchianos recommendation.

Been stuffing meself with loads of garlic and vit C all winter though.