I thought I’d go to my grave without ever learning the identity of Deep Throat, the mysterious source of inside information in the Watergate scandal. When former FBI official Mark Felt was revealed as Deep Throat a few years back, I was delighted to cross one mystery off my life’s list.

And thanks to a new marketing campaign by Gillette, I can now dispose of another: the lifespan of a razor blade.

Legend has it that the blade makers have never revealed the expected lifespan of a razor blade. I always figured they must have secret testing data showing that a blade would last for months and months. Since most men tend to replace their blades every couple of weeks, they’d miss out on a lot of sales if guys knew they could actually get longer life from a blade.

So I was surprised recently when I started seeing a commercial for Gillette’s new ProGlide razor. In it, a handsome young man travels the globe, having exciting, exotic experiences – all the while keeping clean with a single ProGlide blade cartridge.

The voiceover promises shavers “up to five weeks of comfortable shaves” with a single cartridge. (Although each cartridge contains no less than five blades – a phenomenon I wrote about here.)

So, it looks like five weeks has been established as the outer lifespan for the modern razor blade – at least, for men. Other than a one-time stunt in high school, I don’t know much about leg shaving, so I can’t vouch for the female side of the equation.

But one thing hasn’t changed – even though Gillette is broadcasting their blade’s lifespan on national TV, they still don’t seem eager to discuss it. A Gillette spokesman didn’t return my email seeking comment. That’s OK – I’m just happy to have the question settled once and for all.

Now, if someone could just let me know what happened to Amelia Earhart, D.B. Cooper and Jimmy Hoffa.

This is John Reinan’s weekly marketing column for MinnPost.com.