There is an entry in the shave wiki that addresses this, see the link beneath the "de" at the end of Badger & Blade on the upper-left part of your screen.
As you noted, this varies among shavers. I usually get 4-6 shaves/blade and have taken to simply changing every five shaves unless I notice a problem. Some blades seem to be more consistent than others (a problem I notice with the Derby Extra). My personal favorites (so far) are Israeli Personna (RPs), Israeli Blues (Crystals), Gillette 7:00(Yellows), and Wilkinson Sword. I've done some experimentation and am always open to sampling new blades, but that can lead to some disappointments. The two Israeli blades I listed sell for $10 and $12/100 (respectively) from WCS, and are both solid performers, making them hard to beat!
Middle of MO
“Much of the social history of the Western world, over the past three decades, has been a history of replacing what worked with what sounded good.”
"The cost of freedom is eternal vigilance."