I am looking at your rectangular ofuro.
What is the life span? I am concerned that due to its rectangular shape, over time, the water pressure might cause leaking at the seams.
The life span depends very much on the site conditions and care in using it.
A wood tub is built to last 30 years and over of daily use: anyway the average life span is about 10-15 years.
The problems are not caused by water pressure, as we dimension carefully the thickness of the planks and the joints are extremely solid.
If the ofuro is wider than 800mm we use a top beam frame construction.
The causes of aging come from humidity level. If it is too high, it can create stains or mold. If the room is to dry, the wood might crack. Of course cracks can be repaired, but the esthetics is compromised and after 10-15 years many decide to replace the tub.
Keep in mind anyway that ofuros in commercial spas or hotels in japan thus are heavily used, have a long life span because are well mantained.
If you check periodically an hygrometer and keep the habit of quickly wiping the tub after every use with a towel, your tub will be strong and even more splendid after 30 years.
Here to the right is the side board (against the wall) of a 15 years old tub we just replaced in a hotel. Considering that it could not be accessed for cleaning, it is still in good condition after all this time!