If I had a good reason to memorize this, I would have it down within a couple weeks. I'm pretty certain of this, as I do have private keys committed to memory. That's not even impressive, folks who have good memory palace practices could recite that string for you a few minutes after seeing it. I wouldn't expect it of the average user, but it is totally doable for a human of average intelligence -- it's just not something people care about. For long term storage (rather than a repeatedly entered password) it's even easier if you convert[0] it to an equally random yet more humanly memorable line of text: "duck fade ink love reality possible shown screw thing win invisible realize child suffocate somehow petal army nervous"

[0]: https://brainwalletx.github.io/#converter