It'll go to random nodes, and if those chunks have zero popularity they'll get discarded pretty quickly. The exact details would depend on client settings but probably don't matter for a general answer.
For content that is never accessed, you have lifetimes between 2 weeks (bigger than 4 MiB) and 3 months (smaller than 2kiB).
Content that is accessed regularly can stay around indefinitely.
Some of the sites of Freenet are older than a decade and still working well.
To see the actual measurements, you need to run Freenet and access the fetch-pull-stats: USK@lwR9sLnZD3QHveZa1FB0dAHgeck~dFNBg368mY09wSU,0Vq~4FXSUj1-op3QdzqjZsIvrNMYWlnSdUwCl-Z1fYA,AQACAAE/fetchpullstats/537/