You're likely to get just as many conflicting views on a forum.
There are different amounts for different type images, different resolutions, different output targets ( print, web, etc.)
Theres a "gazillion" tutorials on the web--- Just type "Unsharp Mask" in Google----------
Here's one:
http://graphicssoft.about.com/od/sharpening/ If you can afford it , I'd suggest a plugin such as FocalBlade; I use both it and NIK Sharpener Ink Jet Pro.
These plugins use much more than basic USM; combining edge finding, blur, etc., etc. If you do a lot of printing they save a lot of time and hassle, and are well worth the cost.
Leigh