RFC 2326

12.37 Session

   This request and response header field identifies an RTSP session
   started by the media server in a SETUP response and concluded by
   TEARDOWN on the presentation URL. The session identifier is chosen by
   the media server (see Section 3.4). Once a client receives a Session
   identifier, it MUST return it for any request related to that
   session.  A server does not have to set up a session identifier if it
   has other means of identifying a session, such as dynamically
   generated URLs.

 Session  = "Session" ":" session-id [ ";" "timeout" "=" delta-seconds ]

   The timeout parameter is only allowed in a response header. The
   server uses it to indicate to the client how long the server is
   prepared to wait between RTSP commands before closing the session due
   to lack of activity (see Section A). The timeout is measured in
   seconds, with a default of 60 seconds (1 minute).


So when the RTSP server returns:

RTSP/1.0 200 OK
CSeq: 3
Transport: RTP/AVP;multicast;destination=239.1.158.234;port=44078-44079;mode="PLAY"
Server: 0060e057bc25
Session: yzkpnxtlxwrtbgne;timeout=30
Date: Thu, 18 Jun 2015 16:55:01 +0200

it should send keep-alives every 30*0.9 seconds (90% of timeout to be sure).

rtspsrc states:
0:00:00.037728263 24517      0x1f840f0 DEBUG                rtspsrc gstrtspsrc.c:4942:gst_rtspsrc_loop_udp:<source> doing receive with timeout 54 seconds

(still the default 60 seconds).