It would take less than 8 mins. It depends on elasticity of space time fabric. Consider Put a marble on a cloth and then observe how much it descends and curves cloth. Now, suddenly remove marble The time taken by cloth to regain its original position, so that it ends point feel no curvature, clearly depends on elasticity of fabric and amount of depth it had sunk. Our space, it is very much elastic and for curvature of sun, it would take few seconds to regain its original position.
Gravity and gravitational waves are different.
Let me clear it to you. Consider a taut string 100m long. Send an impulse to it. Clearly,it has some speed of propagation, nearly 2m/s(depends on material). Now, cut one end of it, how much time does it take for other side of string to know it.Somewhat 1 second. Isn't it amazing. Update: This example wont work,, since 2 waves are totally different impulse was mechanical wave ,, droping of string is electragnetic wave.😳
This all is just my theory prediction.. No such thing is yet proved..
Though,, exact speed of gravitational wave is imprecisely known comapred to light wave.