I hope someone can correct me if I'm wrong, but I don't know of any feature like that in GrapheneOS.
What I can tell you is that the auto-reboot feature is particularly handy in the situation you're describing. If they get the phone, this will give them less time to extract data when the latter isn't at rest (encryption keys are in memory). This is already something quite hard to achieve without substantial resources.
Now, when the auto-reboot feature takes action, the data will be back at rest. Encryption keys aren't really retrievable, and since Titan M makes bruteforce very difficult (so it strengthens the lock method which remains the main derivation factor), it goes to say it will take an even greater amount of time and resources to get into the phone.
I'd say most users should set auto-reboot to at last 24 hours. It's not a huge inconvenience that way and it can mitigate some attacks. Users with a particularly high threat model could consider setting it to a lower value.