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.
i'll have to disagree with a "Bitcoin has become irrelevant" statement above.
Please don't take it personally but if you can imagine the concept and the scale of this whole project?
~ This is the Bigest Thing what has evolved out of Open Source and happened to US ALL! ~
This is a cure for dying and corrupted planet governed by brainwashed and uneducated individuals.
i promise you that kids of your grandchildren will have difficulty to understand how primitive we were cutting trees to make useless notes calling them money that derives its value from government regulation or law.