You should submit it as soon as reasonably possible after the event. Waiting too long makes it harder to explain the facts, gather documents, and show a clear link between the incident and the costs you are claiming.
There is also a practical reason to act early: some documents become harder to obtain with time. Clinics change systems, providers stop responding, receipts get lost, and memory becomes less reliable. Early submission protects you.
If the situation is still ongoing, you do not necessarily need every final document before making first contact. You can usually notify the claims route that an incident has happened and then continue supplying documents as they become available.