That depends not only on the coil inductance but the amount of stray capacitance in the circuit.I would like to know how much voltage will be generated when we turn off the relay suddenly.
Inductive kickback voltage.
No matter what those two values are, the resulting kickback voltage will likely blow the transistor driving the coil, so you must add a suppression circuit, such as the diode across the coil shown in your diagram.
The diode reduces the kickback voltage to less than a volt.