Not that long - I think the thing you have to do furthest in advance is invite people. Then you can sort food, drinks and games, and she can sort out any surprises for you - it's almost best if the invite comes from her, because then she can put a note saying 'if you're going to bring a present you can call me for ideas or bring money towards a something'. Whereas from you, saying give me presents, doesn't sound as good.