So far I've adopted the simple life by changing blades every Saturday AM, in anticipation of having a "broken in blade" by Monday AM for work.
I also go with the "first bad tug" concept of trashing a blade the first 2 times it tugs too hard on a shave. They're just too inexpensive to dick around with and the cost of messing up your face is not worth it.
Bottom line: once a week or the first sign that the blade has outlived its usefulness.
"...and today I said, "[I][B]let there be lather[/B][/I]", created lather, and there was lather; I saw the lather, and it was good" :a2: