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The delta time between a "TCP Keep-Alive" packet and the response to that packet (a bare ACK) should be close to the RTT of the connection.
The delta time between sending "TCP Keep-Alive" packets is up to you and dependant on what you need to acomplish with the TCP Keep-Alive packets. Originally the TCP Keep-Alive packets are used to clean up connections for which the connectivity to the peer system has been lost.
Nowadays the TCP Keep-Alive mechanism is used primarily to keep the session tables of intermediate devices like Firewalls and loadbalancers fresh. Otherwise sessions that are idle might be removed from the session table. If you use TCP Keep-Alives for this purpose, the interval between the Keep-Alive packets should be less than the idle timeout for that traffic on the intermediate devices.
