Sentience — which means the capacity to feel *something* (anything) — can differ in quality (seeing red feels different from hearing a cricket), or in intensity (getting kicked hard feels worse than getting lightly tapped) or in duration (now you feel, now you don’t).
But the difference between whether an organism has or lacks the capacity to feel anything at all , be it ever so faint or brief, is all-or-none, not a matter of degree along some sort of “continuum.”
Mammals, birds, reptiles, fish, and probably most or all invertebrates can feel (something, sometimes) — but not rhododendrons or Rhizobium radiobacter or Rutstroemia firma… or any of today’s robots.
There is no more absolute difference than that between a sentient entity and an insentient one, even if both are living organisms.
(Sedatives can dim feeling, general anesthesia can temporarily turn it off, and death or brain-death can turn it off permanently, but the capacity or incapacity to feel anything at all, ever, is all-or-none.)