Japanese naval construction is the best around right now. They have the shortest design times and the quickest build times, a product of having one of the three major shipbuilding industries on the one hand and the JMSDF keeping to a consistent yearly build schedule for the past 60+years. If Australia had gone with Soryu rather than the French design they would already have new submarines in service. If MHI says they can build 12 FFMs at once, I believe them.
Japanese surface warships have been built with and have served for a 35 year lifespan for decades. It looks like the Kongos will be the first to exceed that, so they may have been built for a 40 year lifespan.
FFM is replacing the Mogamis, FFM is the current design. The first two are due to be laid down next year. The first Mogami commissioned in 2022. Shortcomings were found in the design, more so due to a decision to increase firepower given the changed security situation than anything else, so the order was cut from 22 to 12, and they were redesigned into FFM, with the first pair (of 12 total) supposed to commission in 2028. Note that means three years from commissioning of one ship to deciding to go with a modified design to designing a follow on ship to cutting steel. Japanese naval construction doesn't mess around.
The DDG(X) design for the USN was first shown in 2022. Japan started looking at sea based BMD in 2020, ASEV for JMSDF was first talked about in 2022, the design fixed in 2023. DDG(X) isn't anywhere near starting construction; the two ASEVs are scheduled to begin building soon, with commissioning scheduled for 2027 and 2028. That's three years from concept to design, and 4-5 more to commissioning, or 7-8 years to commissioning vs. DDG(X) which is at 10 years and probably slipping for even beginning construction.