Post Process Settings wont update gradually

I am trying to update the post process settings on my camera to make the background around an inspected object blurred. I can blur around the object, but I wanted to do it gradually.

Currently, I have this setup
.

And this works perfectly

But I can’t get the blur to update over time because at the moment it is instant and not gradual. Any help would be greatly appreciated.

Thanks in Advance
Harry