The visa process depends a lot on which country you are coming from, and your reasons (ie. job vs. marriage). It can be anywhere from a few months to a year.
If you're looking to getting a green card/become a citizen, try to get your visa and get to america as soon as possible, before your child is born. The US government is very reluctant to separate children from mothers, and if you have your child in the US, they automatically become a US citizen.
