(1) the external, physical duration of a phenomenal experience,
(2) the internal, felt duration of the experience, and
(3) the believed duration of the experience.
To illustrate the differences: Experience machine A gives you pleasant experiences for 100 years. Machine B (allegedly) gives you all the same experiences, feeling exactly the same from the inside, but packed into just a single real-time day. Machine C gives you one day of pleasure, and then simply implants in you the false belief that it felt like it lasted for 100 years.
A key question is whether dimension (2) is stable and independent of the others, i.e. whether Machines B and C are really distinct. But first, we must consider whether qualia (conscious experiences) are to be identified with brain states or their contents:
Our talk of conscious experience risks conflating two distinct objects: the qualities (properties) of the vehicular experience or representation itself, versus the qualities represented in the experience. It's plausible to think that we can access only the latter. For information to be available to us, it must be represented in our minds...
This includes experience itself: there may be facts about it of which we remain unaware. This is plausibly true of the temporal properties of our experiences, for instance. It seems to me that I experience A before experiencing B, but in actual fact all I have access to is my mental representation as of "A followed by B"... I don't really have a "mental eye" in my head to tell me what's going on in there, independently of what eventually enters into a conscious representation. Our introspective capabilities are hence severely limited.
Clearly, the brain states themselves last only a day in Machine B's case. But they represent 100 years' worth of happenings. How are we to describe this? Here are two options:
(i) What you experience is the brain state itself, and hence lasts for only a day; it's just that you're left with the false impression that it lasted longer than that. Machine B was misdescribed: in fact it is no different from C.
(ii) What you experience is the content as represented by the brain state, viz. 100 years. It is truly the same content/experience as given by Machine A; it's merely the physical bases of the experiences which differ, and that's nothing to care about.
I'm inclined towards (ii). But if experiences can so radically come apart from their underlying brain states, we may question whether Machine C is actually as deceptive as we'd assumed. If the implanted belief changes the contents represented in our phenomenal states, perhaps this actually gives us 100 years of experiences?
One reason for doubting this is reflection on the paucity of the representation. It's one thing to write a story which says "100 years passed", and quite another to fill out the details for 100 years' worth of fictional events. So, if we think of Machine A as taking a long time to impart rich informational content (lots of pleasure), and Machine C as taking a very short time to impart a very thin representation (very little pleasure), the question whether Machine B imparts a lot or only a little pleasure comes down to the richness of the representations it implants. Sound right?