I start building them as soon as i get them too, but i usually have an "order" that things are built in, so when i first get it i build it in my cities that dont have anything to build, so they have a bunch of production, and i usually have a sewer system in the city before the factory, so its a sizeable city with a bunch of production. Id have to say that if i had to make a numerical choice, i would go with 8-10. it seems like a good number, not to high, not to low