Unfortunately it depends on the baby. I have friends whose children were sleeping through the night by 3 months, but my son is 16 months and still wakes up sometimes. Also, whenever there is something new (teeth coming in, vacation, milestone - such as learning to crawl, sit up, etc) their sleep patterns might change.
Things you can try to get them to sleep through the night:
a white noise machine/soft music to help cut any disturbances
make sure they are not too hot or cold
make sure they are in a clean diaper
make sure they are fed/not hungry
