J.G.
I think if you can afford it, one week's salary is extremely generous. If you can't afford it, just give what you can.
We have a full-time nanny, and I can't afford to give her an extra week's pay. I'm giving her 2 weeks off at the end of December (which she still gets paid for), and I'm considering that her 'bonus'. If I can, I'll probably give her $100 to $200 on top of that. I'm also planning to have my boys pick something fun out for her at the store (nothing expensive) and do some kind of crafty project we can give her.