It depends on some places, they have to know right away. I know where I live they can let you go if you don't let them know by 15 weeks. I'd just tell them right away so they can get prepared in advance.
I'm certain they will get you to do more light things if they have available. What is yyour duty? I wouldn't tell them till my second trimester but if you're doing heavy lifting then you should notify them immediately.