Slimes usually are immortal creatures on fantasy stories, on the sense that they can live forever. But they're not invulnerable, far from It actually, they're famed as the "weakest" monster because they usually are very fragile.
But Rimuru got some super OP skills from "eating" Veldora, as well as a huge boost on It's overall physical capabilities and magical power from receiving a name from It. After that It only kept getting stronger from every monster It ate, specially the Orc Disaster and the Charybdis. So at this point, considering how fast It regenerates, I think that unless It picks a fight with someone MUCH stronger than It, Rimuru should be pretty much immortal. o: