table lifetime command

I am not well versed in the use of table commands.  What I would like to do is "terminate" any session to a VS after 15 minutes regardless of the session being idle or active traffic being passed.

What I have tried is:

when CLIENT_ACCEPTED {

    set tbl "virtip:[IP::local_addr]"

    table lifetime $tbl 900

log local0. "ltime:[table lookup -subtable $tbl lifetime]"

}

The logs show the "ltime" as empty.  My question is, what is the correct syntax to set the lifetime (if this is the correct variable) and to generate the log entry showing the lifetime value.

Thanks,