It still supports the pixel 4, which was released 4 years ago. Google stopped supporting it a year ago.

If google does maintain their stuff properly for 8 years, that means grapheneOS will also easily do 8 years, maybe even more.

graphine usually gets updated for as long as Google is still releasing patches, now of course graphine is a FOSS Project so zero guarantees of any kind

For as long as the phone is getting security updates from Google.