Im usually given enough to last me 2 weeks when I get samples. Only because all the MAs at my local counter know me and know how sensative my skin is. It usually takes a week for a serious reaction to show with me and when I say serious I mean it.
They never used to though, they used to give me enough for 2 - 3 uses. But one time last year I went in, got a sample of the tinted moistureiser, used it for 3 days and bought it. A week later I had to return it because my skin was swollen and puffy, the skin around my eyes and lips was chapped and cracking and I was a mess. It was the moistureiser, so after that the counter manager sat all the MAs down and told them that when I ask for a sample, give me enough for 2 weeks.
So now when I go in and ask for a sample, I get a lot, and if its a new MA they ask my name. I guess the new staff are told to ask names before giving samples so they know how much to give certain customers like myself.