I only finished it once, imported my save from the first game: fem shep, whatever the class for a mix between guns and biotics is called. I finished it in about 30 hours.
Gonna play it through again soon though! I hardly ever do that anymore but it is an excellent game. Miles better than the first game which I could barely continue playing but I forced myself to.
Now if only Valve would release some stats for how many times people have finished Half-Life 2.. I am fairly confident nobody in the world has finished it the amount of times I have.. Back when I was a sad teenager with no life I finished it around 35 times before I stopped counting, and since then have finished it a few more times... Somewhere around 40 times overall. I'm not really proud of that =P