Forever. I just bought some overpriced Gillette's & I'm glad I did. I got a big box of the Dorco razors from Slickdeals (the ones that Dollar Shave Club apparently uses) & they are like cheap windshield wipers...just not as smooth as the good brands. Tug on your skin, kinda hurts, just don't do as good of a job. Went back to Gillette after a month of using the Dorcos, soooo happy I did. Worth the money!
Also learned a good tip from the Slickdeals Dorco thread - dry off your Gillette after every use & it can last up to a year! I've been doing that and the blade has stayed good so far, which is awesome. And if you think about it, the Gillettes are really only expensive in your head...$20 for a 4-pack, and a blade typically lasts what, at least a month? So you're really paying five bucks a month for a sharp razor. I can live with that. It IS annoying when you buy that tiny little package for so much money at the store though