I believe it says on the site '100% quality control at the assembly line level: each block is factory tested to 25 psi (1.7 bars) to guarantee zero defect. '
So I guess 25 psi is fine

but probably a bit more than that. Typical engineering is to have a safety margin significantly larger than what its tested to, but considering this is a water block, and if you see 15 psi your rad will probably die, it might not be too far off.