Noah spent almost 100 years to complete the building of the Ark. It took that long because of the nature of the conditions it would endure. God had to tell him how the Ark should be built because He knew the conditions it would endure, so the ability of the Ark to withstand the intensity of the rain successfully resulted from Noah doing as God instructed.
Knowing what God has called you to do isn't enough to be successful at what He has called you to do. You also need to wait on Him for instructions and directions on how He wants you to do it. The 'how-to' makes you successful with what He has called you to do. I knew what God had called me to do at an early age, and everything after that was Him guiding me on how to do it. God is more interested in the 'how' because it determines the success of what He called you to do. This is why He gave Noah detailed building instructions after choosing him to build the Ark.
You can't be successful at what God has called you to do by doing it your way.
Many people know what they are called to do but are executing it their way, and it seems like they're failing at it. The Bible advises us to trust God and not lean on our understanding. Every device you purchase always comes with an instruction manual. The manual is there for you to know how to use the device successfully. Every calling of God has its unique manual, but we struggle with successfully executing what He has called us to do because we ignore the manual because we believe we know how to operate in what He has called us to do.
How to successfully run a business is what business owners look out for. They fail multiple times to acquire this knowledge. Another reason why God gave Noah detailed instructions was because He couldn't afford the Ark to fail at doing what it was supposed to do, which was preserving the lives of those in the Ark. God can't afford you failing because of the lives and destinies that are connected to you successfully walking in your calling. This is why those whose calling is great, He can't let them do it their way.
Also, He gave Noah those instructions because He had a plan of when He would do what He wanted to do, so He needed Noah to work with His time. Noah didn't have to think of the measurements or materials to use. The unit of destiny is time. God needs you to follow His instructions to work with His timing.
In conclusion, Noah used nearly 100 years to complete what he was called to do. You don't determine how long it would take to complete what He called you to do. It is in the hands of God.