I know that the height will vary based on how high the sink is mounted and how deep it is, but I'm trying to figure out how long of a run I can make for my sink.
The trap is usually attached to the end of the tailpiece, and tailpieces are typically between 6 - 8 inches long (though I've seen up to 12). So the "standard" height of a trap would be, 6 - 8" below the fixture.
(P-trap is the area between the two horizontal washers)
This question doesn't make sense because you will buy the ptrap and its size depends on the width of the pipe you pick. Look to the picture above for normal dimensions.
As for the fixture outlet, the length from your sink to the beginning of the ptrap... Well this doesn't really matter and is totally dependent on the trap outlet height. This could be 6 inches or 2 feet. Depends on where your trap outlet comes out of wall (or meets below floor).
The trap outlet distance is regulated based on the pipe diameter - and I am sure this has been answered many times on here.