The installation torque recommendation for almost all square taper cranks is in the 30 to 35 lb-ft range which is WAY more than you'd expect and more than nearly anyone can apply with a standard L-shaped allen wrench. Turbineblade is either very strong or has been lucky.

Torqued properly, a square taper crank should never loosen on it's own.

BTW. if your current crank has come loose and been retightened, it's a very good bet it has been damaged beyond use. Once the square socket in a crank arm has been distorted by being ridden loose, it will never fit properly again and must be replaced.