Until the host takes his off???

Seriously though, I was taught that the tip of the tie should come no lower than the middle of the belt buckle, and no higher than the top edge of the lower garment (be it trousers or kilt). That's generally about a half inch to an inch of leeway (not a lot). With a waistcoat, the tie should not be seen below the bottom of the waistcoat. (Ideally, the top of the lower gament shouldn't be seen either).

A guideline I have found is to use the stitched seam crossing the width, near the middle(not the one running down the length), if your tie has such a seam. Tie your knot at or just above/below this seam, and the tie should be close to the right length.

The type of knot used is dependent upon the style of collar, each knot being a different size and therefore taking up more or less space and tie length. I won't describe how to tie each one - that can be found elsewhere. Straight/point collars get the four-in-hand (a roundish, narrow knot). Button-down collars get the half-Windsor (a bit larger and more triangular in shape). Spread collars get the full Windsor (the largest of the three basic knots, and squarish in shape).