Hi,
As far as publicly documented behavior goes, Microsoft Graph does not provide a guaranteed or officially documented TTL for mailbox delta tokens. From practical experience and community discussions, the lifetime appears to be implementation-dependent rather than a strict published SLA.

A few important points:

• Delta tokens should generally be treated as opaque and potentially short-lived.
• There is no confirmed fixed expiration window published for mailFolders/messages/delta.
• The backend may invalidate tokens because of mailbox changes, service maintenance, topology changes, retention boundaries, or internal cache/state eviction.
• So yes, in practice, expiration may occur independently of elapsed time and not purely because of a fixed TTL.

Because of this, production systems at scale are usually designed assuming that:

• Any token can eventually become invalid at any time.
• A 410 Gone is a normal recovery scenario rather than an exceptional edge case.

To handle this reliably at scale, common strategies include:

• Staggering synchronization schedules to avoid synchronized token refreshes
• Applying exponential backoff and retry policies
• Limiting concurrent full re-sync operations
• Maintaining checkpoints/watermarks to reduce resync scope where possible
• Using queue-based resync orchestration instead of immediate mass retries
• Monitoring token invalidation rates as an operational metric

For very large-scale systems, it is generally safer to architect around eventual token invalidation rather than relying on a stable long-term retention guarantee from the service.

Hopefully Microsoft can provide more authoritative clarification, but currently the documentation intentionally seems to avoid guaranteeing a deterministic TTL.