For my tastes, just the minimal necessary to form a patty. You should still see the threads from when it went through the grinder on the surface of the meat when done forming the patty.
For best results, the patty should be at room temperature before cooking. For highly compacted patties, this isn't much of a problem because they are so dense the heat conducts into the middle quite easily.
Note that the patty is inevitably thick than what you find at most places. When the patty is finished cooking, it will need to rest for three or four minutes before preparing the hamburger.
[This message has been edited by eric76 (edited 7/4/2011 10:41a).]