The STOS has an excellent grind right out of the pack and I wouldn't figure on losing much of anything on those. Tuskers and other heads that need more work will lose a little weight in the sharpening process, but you'll never notice the difference if your bow is tuned.
But to answer your question, to sharpen up a double bevel head, generally I lose 5-10 grains. If I make a double bevel Tusker a single bevel, it's usually 10-15 grains. Again, in actual application you can essentially ignore the difference.