The phrase "ascertain when to" is correct and usable in written English.
You can use it when discussing the need to determine the appropriate timing for an action or decision.
Example: "It is important to ascertain when to implement the new policy to ensure a smooth transition."
Alternatives: "determine when to" or "figure out when to".
8 9 With respect to the possible infection period in the second patient, we cannot ascertain when person to person transmission occurred.
Despite this, ophthalmologists often found it difficult to ascertain when it is appropriate to certify patients, particularly for people with long-term conditions.
Early conversations between patients, caregivers and health professionals are encouraged to ascertain when cancer patients may want to consider treatment limitations to avoid inappropriate and aggressive care at advanced stages of illness [ 2, 3].
Experts have been unable to ascertain when the Kaigetsudō studio ceased to exist, though little is traceable to it beyond 1716.
In designing a therapeutic strategy to prevent disease recurrence, however, it is necessary not only to know the total risk of relapse but also likely to ascertain when recurrence is most likely to occur and when the risk becomes minimal.
The Japanese say that they are hunting whales off Antarctica in order to ascertain when there will be enough to harvest for profit.
Plus, it is virtually impossible to ascertain when the studio disburses payouts to its "partners," including director Jerry Bruckheimer.
It isn't easy to ascertain when Baroness Thatcher first referred to her minimal sleep schedule, but the figure of four hours has passed into lore.
Potency differences for predominantly chrysotile versus amphibole asbestos-exposed cohorts become difficult to ascertain when meta-analyses are restricted to studies with fewer exposure assessment limitations.
