There aren't any rules on this. I guess it depends on things like: what the mother feels comfortable with, if the child is breastfed, who the child will be left with, etc. I've found that things affect us, as mothers, a lot more than it actually affects our children.
When I left my son for the first time, he was 7 months old. His dad & I went to Jamaica for a wedding, and were gone for 5 days. I missed him TERRIBLY!! My mom came to our house and stayed with the baby. I figured if Mommy and Daddy would be gone, he could at least be comfortable in his own home. I called often and my mother texted pictures, and when I realized I was having a more difficult time than my baby, I loosened up a little bit. Just a bit.
The second time, we (his dad & I) went on a cruise with some other family and friends. My son was 13 months. Again, my mom kept him, but this time I allowed him to stay at my parents' house. This time, we were gone for 6 days, I think. He was fine.
In February, I traveled to Orlando for work. This time, I took him with me. Dad stayed home. During the days we were there, though, my aunt took care of him while I attended a conference. This is not a person he was familiar with. He did have his fussy times, but all in all, he was fine. He was almost 18 months.
Lastly, in March I went to Miami to celebrate my 30th birthday. My son stayed home with his Dad. I was gone for 4 days. My son was absolutely fine. He was just about 19 months.