With each of my five pregnancies, my morning sickness started between 5-6 weeks from the first day of my last period, and lasted anywhere from 4 to 6 months.
With my first, I was working full time and under a lot of stress at work. I had nausea and vomiting for about 6 months, and then got a little relief, but still was slightly nauseous all the time. I can even remember vomiting 2 weeks before the baby was born, and vomiting during labor too. It did get better, though...
With my second, I was home full time, had a 1 year old, and was way more relaxed than during my first pregnancy. I had morning sickness for about 4 months, and then it was done.
With my third, I was home full time with 2 children, but was also a day care provider for 5 other children. My stress level was slightly higher than with #2, and my nausea seemed to last for about 5 months. But then it was over and I could enjoy the rest of my pregnancy.
With pregnancy #4, I was home full time and homeschooling 3 children. My nausea was probably about the same as with #3.
Then there was my 5th pregnancy, and the nausea and vomiting was way more pronounced than with the others. It started at 5 weeks, and lasted a good 5 months, and was so bad that I could barely get off the couch. There was very high stress in my marriage, and I believe that contributed to the severity of the nausea.