The limit is our distance from the Sun. After a certain point the greenhouse gasses can't make up for the fact that we just get less radiation than Venus does. The maximum potential I've seen is 10°C - almost all life would go extinct and we'd have to live on the tropical Antarctic archipelago, but not Venus.

Everything find equilibrium eventually. I'm sure any limits for a runaway situation depend on a lot of factors, but their ceilings are all far above anything we could tolerate. Runaway doesn't mean there's no point to level out, only that at the time it's not controllable and escalating fast.

The last "runaway" situation the Earth had was called the Paleocene–Eocene Thermal Maximum (PETM) 56 million years ago and globally had a 5-8 degree Celsius rise over thousands of years. That might be a good example of a natural situation and its limits. Keep in mind the differences in rate, we're increasing the global temperature faster than the PETM (or anything we've found in geological history) so we don't know how that faster rate will act in determining a peak. There's theories of pushing the Earth into a hothouse world that would have its own equilibrium that is far hotter than we can survive.