The mathematical equation is I²t = constant.

And generally the time tripping is given at 6 Ir.

For example the Micrologic 2 for NSX gives the tripping time 16s at 6Ir.
Then what’s about time tripping at 2Ir?
I²t = I’²t’
t = (6/2)² x 16 = 144 s

This mathematical relationship is available only in long time delay of course when you reach
the Isd (short circuit) threshold it is no longer available.