That depends a lot on the paper and also the pen. I wouldn't aim for decades with just any random fake sharpie + that free college block with a company logo that you got in a hand out...
I still have notes which I wrote over 20 years ago, which were written with a cheap pen on ordinary note paper.
My mum has diaries of my grandmother which were written before the war. All still perfectly legible.
Now if you want to preserve the private keys for CENTURIES it's of course different. But I think by the coins will have either been long spent by your descendants or Bitcoin has become irrevelant.
