About 25 years of which were spent on debating whether an N-sub was required in the first place.
And the answer was yes. Without SSBNs, a nuclear deterrent has no credibility unless you want to either move missiles periodically on trucks and trains or keep airplanes with nukes in the air 24/7. The problem is that SSBNs are extraordinarily expensive to build and maintain. And you have to build them quiet so that they aren't followed. Good luck to India on the endeavor. I just hope they have the security procedures in place to prevent this technology from leaking to Pakistan. The last thing the world needs is for Pakistan to have the technology to build a blue water sub that can deploy nuclear weapons around the world.