"Forever" unless the knowledge is out of date, or you don't use it and forget, or until Alzheimer's or some other dementia happens and everything disappears (or until your death, I suppose, but gold also tends to change hands around that time).
Do you always succumb to the need to publicly police people? The other person just voiced what I thought as well, that knowledge definitely isn't forever. Why not comment on the fallacy contained in the meme?
Ngl, I did not actually expand the picture to see the caption. I just saw the main image about knowledge. Post-docs absolutely get fucked and I do feel bad for them.
I quit one year into my PhD because I was miserable in grad school. I have to assume most post-docs have some crazy resilience or some kind of mental issue/trauma that keeps them going and going. I guess that's probably true for most people but post-docs do some seriously hard mental work and basically get shafted the entire time (at least in the US).