availability group loses connection to secondary server

I have a SQL 2014 SP1 set of servers with two asynchronous copies of an availability group.   One of the asynchronous sites is down and SQL can no longer replicate the changes.  I need to understand how long SQL Server can continue this way before the secondary replica will no longer be able to catch up.   I assume this is really tied to the transaction log on the primary replica but would like it clarified.

Thanks