As far back as you like. A few days ago someone posted some observations from 24 years ago in a project I run.
I’ve posted photos from the late 90s/early 2000s. The main issue I have with adding my older images is keeping track of what’s been added and what hasn’t been added.
There are a few floating around on iNat from the 60s and earlier.
There are a couple of past discussions about people’s oldest observations that are worth digging through for specific examples of old observations: