How hard do you have to hit someone for their arm to blow off
Depends on the durability of the person whose arm is blown off.
In fact, in order to make Saturn someone who can be beaten, the durability has to be low, otherwise it doesn't really make sense; if you had Kaido's durability with Saturn's regeneration, it would probably be unbeatable.
Of course, there are cases like Gotou from Parasyte, who has a crazy durability (for his own verse), with a crazy regen power, but with very slow regeneration.
I think that Saturn is the first case, very low durability, but crazy regen power/speed.