ICAO documentation is normally restricted to recommendations and standard practices from which national authorities can develop specific requirements.
e.g. for EASA … at least 100ft, but not normally more than 300ft - depends on type of system (aircraft), failure rate and duration of exposure (time ~ height).
CS-AWO 312 Alert Height
For a fail-operational system with a decision height below 15 m (50 ft) or with no decision height, an alert height must be established in accordance with CS-AWO 365(a) and must be at least 30 m (100 ft).
AMC AWO 312 Alert Height
It may be operationally useful for the alert height to be somewhat higher than 30 m (100 ft) since this would permit reversion to a higher decision height in the event of system failure. A maximum value should be established during certification and it should not normally be above 90 m (300 ft).
CS-AWO 365 Fail-operational landing system (Automatic or Hybrid)
(a) For a fail-operational landing system, the probability of total loss of the landing system below the alert height must be Extremely Remote. Demonstration of compliance must be by means of a suitable analysis programme supported, where necessary, by a simulation and flight test programme.
Special precautions must be taken to ensure that redundant sub-systems are not vulnerable to simultaneous disengagement or failure warning.
(b) A fail-operational landing system must operate as a fail-passive system following a first failure, which leads to loss of fail-operational capability.
(c) A fail-operational automatic throttle system must be provided unless the effect of loss of automatic throttle control is minor.
https://www.easa.europa.eu/sites/def...r%20CS-AWO.pdf