Subscribe to Pub/Sub with expiration

I can't figure out what I need to do to subscribe to a pub/sub node but subscription with an expiration date time, so in case my browser closes my subscription will expire.   What is the option I will need to set and format? 

It says uses expiration:

Ignite Realtime: All About Pubsub

Additionally, does open fire clean up expired pubsub subscriptions or will i need to do that manually at a later time?

Thanks,

Morgan