Since you already provide the Link to wowhead in your post and oddly it's data is very limited I will provide information from wowdb:
There are currently ~150k kills of Mobs listed. The best dropchance as per site has
Spitebone Guardian and
Shadowy Attendant however it only was killed about 4k and 17k times. If we put all this information together the overall dropchance should be around 0.02%
As you can see at the source provided nearly every mob inside Shadowfang Keep is listed. However the site can only list kills and drops made by people with a certain addon to record and transmit these data. You can assume that every mob can drop the sword.
With the limited Data provided we can only assume this but it looks like there is no difference for trash and bosses.
I do not have an exect number of mobs inside sfk, but let's assume there are about 300. With our gussed drop rate of .02% that would mean about 15.000 Kills needed. Which would be ~15 runs.
But....as we all know statistics sucks. This does not mean that on run 15 you will get a drop. Only that the chance to get a drop is now higher then it was before. As live goes it might be you never see it drop. One of the most wanted farm able mounts is Ashes of Alar. Some friends got it on there third kill. Some at 30. Some are over 300 kills on multiple charackters and still did not get it.
As an advice from someone who farms rare mounts. Do not keep those numbers in mind. It is all about luck. You only get frustrated if you do not have your drop by a certain time but since it is not guaranteed chance just keep going on, listen to some music or watch tv and just rush brainless through their and be happy once you loot it.