Hold-up time in SMPS
There are standards that indirectly define hold-up times.  

The standard IEC 61000-4-11 Voltage dips, short interruptions and voltage variations immunity tests indirectly defines hold-up time when the product has to satisfy performance criteria A.
This document shows some tests (e.g. voltage dip of 40% of 20 ms duration or voltage interruption of 60 ms duration).

For medical devices, the standard IEC 60601-1 applies.
See this document for some durations of voltage dips and interrupts.