Forever, essentially.
Ok, not quite.
But illustrations can take a looooooong time, depending who your publisher selected to illustrate and the medium the artist uses.
My mother has two picture books under her belt. The first one took 5 years, the second one will take a year and a half (it's with the illustrator right now). So it depends ENTIRELY on the illustrator.
I'd say the range starts at a year or year and a halfish. And goes up and up and up.