Coming to terms with the concept of "eternity" is not hard here insofar as it is unlikely that someone who does not seek to participate in the world need not experience any moments at all; they may simply exist "between thoughts", serialized as data for much of that time.
Provided the architect of the virtual space permits such adjustments, the protagonist remains subject to their design and control, lacking autonomy over their experience of eternity. Consequently, the prisoner might not even be able to sleep unless the architect specifically allows for it..
Once time isn't quite as much a tyrant over our opportunities to think, once it no longer binds us to one second per second, eternity changes to something else.
The phrase 'The light shifts in virtual space, in seconds, indicating passing days, months, years?' was meant to illustrate how time becomes uncertain, maybe even pointless. My novice writing skills might not have captured this concept well. I could blame the AI for that one but I know better.
In reality, the experience of such a prisoner would be that in the course of a single experiential afternoon three different people walk in, each more alien than the last. They would be experiencing time the way someone traveling the stars might: as an instantaneous journey from their perspective.
Time in a virtual environment can be dynamically adjusted to progress & even regress at varying speeds. I'll also check out 'Altered Carbon.' Hopefully, I can find the time to do so, as it feels like time has been flying by exceptionally quickly lately.