Just remember all inflatables have a limited lifespan, depending on storage conditions this can be 5-10 years but probably closer to 5 years. The glue used to assemble the boards deteriorates and the board falls apart. I have had this happen to high end isups so I guess the cheaper ones go that way too.
Happy to be proven wrong.

Note that they are now available (2021) model in a new technology, with a flat weave in the skin and oblique threads for even more rigidity and less weight. Details on the Gong site.
Note that not all inflatables are made of the same quality material. For instance the double skin that was used by quality brands like Gong was 20% lighter for the same strength than the average one (more structural weave, less PVC filler), and much stronger than the second choice one on no-name brands.

airsail: inflatables - in most brands - progress every year, for instance a lot of glued parts are now welded, so your experience with 5+ years old boards is probably not exactly transposable to modern boards.

Disclaimer: I am a Gong ambassador.

It is in the user's guide flyer downloadable from their web page. It states between 15 and 20 PSI.

I know they are inflated to 33+ PSI at the factory for the QA tests.