OpenSSH: ssh-add / ssh-agent Command Set Maximum Lifetime In Seconds

I'm utilizing a Mac OS X and combination of ssh-agent+ssh-add to adds RSA or DSA identities to the authentication agent. ssh-agent gives me a secure way of storing the private key. However, I'd like to expire identities additional to the agent within half an hour. How do I set lifetime of identities additional to the agent under Unix / Linux / BSD / Apple OS X operating systems?  See More..