Every pregnancy is different, even for the same woman. Ordinarily, some degree of sickness (ranging from loss of appetite to vomiting) occurs in the first trimester and eases up after that.
With one baby, I just kind of had no appetite the whole time. Figuring out what I could eat was a challenge. With the other one, I wasn't generally sick most of the day; but I'd vomit every day at 3:00 p.m. (like clockwork), and that lasted until he three weeks before he was born.
If women get too sick too often they need to tell their doctor, because dehydration can occur.