Occasionally connected hybrid Active Directory

I'm to assist someone to upgrade the hardware on a yacht. They currently have Active Directory, running on a Server 2008 machine installed on the boat.

They also have an Office365 subscription, with a number of hosted email addresses.

I plan to consolidate these two, and setup hybrid AD.

My concern is with the occasionally connected nature of the boat. I remember in the olden days that should an AD server in a forest not sync for 90 or 120 days, it would "tombstone", and would need to be massaged back into the forest. The nature of connection on the boat (if at sea) would mean that there could be extended (i.e. 60+ days) where there would be no internet connection.

How does a hybrid topology handle this? Is there a max number of days that an AD server can be disconnected from the internet before it tombstones?

If I'm understanding what you're trying to do, you may be misunderstanding the nature of how Azure AD works.

Assuming you mean that you want to sync your on premises AD with Azure AD/Office 365, Azure AD isn't a traditional AD like you would have on premises. There are no Domain Controllers that are in any way a part of your on premises AD. Azure AD is not an extension of your on premises AD. If your on premises Domain Controller were disconnected from the internet for an extended period of time that would have absolutely no impact on Azure AD. The only impact to you is that any new on premises objects, changes to existing on premises objects, and on premises password changes (assuming you'd be using password hash synchronization) wouldn't sync to Azure AD until connectivity is restored.

I've seen clients that were disconnected from Azure AD/Office 365 for more than a year, with only the issues I described.