I hope to prolong it as long as possible.
But I have no intention of trying to make it happen or of actually lying at this stage - if DD (almost 11) asks, I will answer pretty truthfully, but still about the magic of Christmas etc.
But we will still have fun and magic even when its obvious that there is no more FC visiting. DD is an only child so no one to keep up the pretence for (DH and I are both the eldests, so did have that).
However, while there may not be a specific big present in the mornings once it is accepted openly (we know she knows, and we think that she knows that we know she doesn't believe anymore - this year was a much more obvious request for things than last year!). But I still intend to do a stocking for her every year, with fruit, a book, sweets and some fun stuff. The difference will be that I can finally start doing the stockings for DH and I that I have planned for years too!! 
The idea will be that anyone in the household can put things into any stocking, and we all have some fun bits on Christmas morning.
We always had a present from us under the tree with presents from everyone else, which get opened much later in the day. And things evolve as DCs grow, but that doesn't mean that it's not still magical - just in a different way!