How long did it take to build a atomic bomb?

Answer 1

It took the US 2 to 3 years to build the industrial infrastructure needed to begin building Atomic fission bombs.

When the US first started building bombs in those factories in 1945 they could build 3 bombs a month with the expectation by the end of the year of building 7 a month. You compute the time per bomb. But remember this is just beginning production, with each bomb hand assembled.

These bombs were delivered by the factory to the field as a kit of parts in multiple crates. Assembly of a MK-III Fatman type bomb from this kit took 3 days in the field.

At the peak of production both the US & USSR had the capacity to build 100s of Hydrogen fusion bombs per day! You compute the time per bomb. This was full assembly line production.

These bombs were delivered by the factory to the field fully assembled and ready for use.