First they have to get pollinated!
Squash plants have separate male and female flowers. They are easy to tell apart, because the female one comes with a little embryo squash already present behind it.
It is common for squash to only produce one gender of flowers at first (usually only male, but it can be the other way around). So no squash will get started until there are both male and female flowers. And then the pollen needs to get from the male flower to the female, usually with the assistance of bees. So you either need to have bees or other pollinators in your garden or you have to help out with hand pollinating.
Once the female flower is pollinated, it will drop off and the squash will start growing. If it is not pollinated, it will drop off anyway and the embryo squash may grow a little bit, but will get an inch or two big and then shrivel up and drop off. If all is going well, once the fruit is set and growing, a week or two to pickable size, depending on weather, etc.