It is up to the employer as to how long they want to set their waiting period so if you are starting a new job, pay attention to when your new insurance starts. Since waiting periods can be anywere from a couple of weeks up to a year, you will want to make sure you keep your health insurance from your previous employer (which can be done throught COBRA) or obtain individual health insurance while in your waiting period.
An important point to remember with employer waiting periods is that the time one has to wait for their insurance benefits to start cannot be in addition to a pre-existing condition waiting period. So, if someone had to wait 2 months for their insurance to start and they had to wait 2 months for coverage on a pre-existing condition then they would only wait a total of 2 months and not 4.

