How does a client access server know that a mailbox database has failed over from one MBX server to another and know to proxy RPC traffic to the MBX server now hosting the mailbox?

If I failover a mailbox it takes outlook around 15 minutes to reconnect and acknowledge the failover and traffic is still being proxied to the MBX server without the mailbox database mounted on

Hi ,

Based on my knowledge , cas server will understand that by querying the active manager.

https://technet.microsoft.com/en-us/library/dd776123(v=exchg.150).aspx

what is the general time it should take to proxy to the correct location?

I suspect there may be an AD replication delay/lag if it takes 15 minutes?

Hi ,

Microsoft exchange replication service is responsible and also it will periodically monitors the health of all the mounted databases.Based on my knowledge because of AD replication delay definitely cas server will not miss the information about active database copies.So please make sure the Microsoft exchange replication service is running perfectly without issues.

On the above statement i have assumed your active directory site to site replication is on delay but without issues.

For your reference : 

Nithyanandham is correct. This information is stored in the cluster database, not in active directory. This database cannot be out of date.

However, CAS keeps a cache of database location. This is updated when a request hits an error.