Suprisingly, this isn't a retcon or an attempt to explain Threepio's throaway line ("In his belly, you will find a new definition of pain and suffering, as you are slowly digested over a thousand years.")
In the film's official novelisation, released at the same time as the film and hence considered 'G-Canon', we get this piece of exposition:
Lando closed his eyes and tried to think of all the ways he might give
the Sarlacc a thousand years of indigestion. He bet himself three to
two he could outlast anybody else in the creature’s stomach. Maybe if
he talked that last guard out of his uniform …
So he clearly feels that survival is a matter of more than mere minutes.
Later sources have elaborated on this extensively, creating an entire ecosystem to explain that the Sarlacc releases nutrients to keep the victim alive.
After being swallowed by the tongue, the victim made its way into the
sarlacc's stomach to be digested, purportedly being kept alive and
slowly digested for a millennium. A strong network of vessels inside
the stomach punctured the victim's skin and muscles and then embedded
itself into victims before injecting neurotoxins into them, preventing
the victims from escaping and ensuring that they remained immersed in
the acidic fluids in the stomach, and attached to the walls of the
The vessels also provided victims with nutrients to keep them alive
while they were digested in agony; sometimes when a victim was in the
stomach for a long time period, the sarlacc actually embedded it in
the lining of the stomach to make room for other victims it swallowed
and to make the stomach stronger so that other victims could not
escape participially once their digestion was complete
The way in which it keeps its victims alive beyond their allotted lifespan isn't covered in any great detail but we do learn that there's a telepathic component to its torture and that it enjoys the pain of its victims, explaining why it would want to keep them alive rather than merely consuming their nutrients.