This is the clearest example in the standards of a requirement that is one sentence long in the CFR and several pages long in practice.
What the regulation says. Where the eyes or body of any person may be exposed to injurious corrosive materials, suitable facilities for quick drenching or flushing of the eyes and body shall be provided within the work area for immediate emergency use (29 CFR 1910.151(c)). Three things are in that sentence: a trigger (exposure to injurious corrosive materials), a location (within the work area), and a quality standard (suitable, for quick drenching or flushing). Nothing about minutes, gallons, or tepid water.
Where the numbers come from. OSHA does not enforce ANSI Z358.1 as a standard, but it uses it to decide whether a facility is "suitable." In its November 6, 2020 letter on eyewash for bloodborne-pathogen splashes, OSHA cited Z358.1's guidance that eyewash facilities should deliver flushing fluid at not less than 1.5 liters per minute for fifteen minutes, concluded that four-ounce bottles are insufficient, and allowed that a one-liter bag of saline "may be more appropriate" only until a proper 15-minute facility is available. Its January 11, 2024 letter on formaldehyde exposure repeated that bottle-type eyewashes are not an acceptable alternative to a 15-minute facility, while accepting portable units that provide 15-minute flushing capability.
Substance-specific standards add their own. Several Subpart Z standards require eyewash and quick-drench facilities outright for their substance — formaldehyde and bloodborne pathogens among them — and those requirements sit on top of 1910.151(c). The construction equivalent is 29 CFR 1926.50(g), worded the same way.
So the honest answer to "is 15 minutes the law?" is: the law says suitable; OSHA has said, repeatedly and in writing, that suitable means Z358.1. Treat the number as the enforcement reality, and read the letters if someone argues the point.