Welcome to FreeSWITCH!
First off, let me just mention that this question is probably better suited to our -users list. (This is the -dev list which has a smaller audience and is reserved for more intense discussions.) If you don't get a satisfactory answer here then re-post on freeswitch-users list.
Now, as far as your question goes... you have different options available to you but it really depends on how you are generating your calls. Also, do you want the call to last more than 30 seconds and have only the recording be 30 seconds? Do you want recording to start on early media or on answer?
As an example of how to do a simple 30-second recording here's one that when you call it, it will start recording, play music for 30 sec, then end the recording and disconnect:
<extension name="record 30 sec of MOH">
<condition field="destination_number" expression="^732664$">
<action application="answer"/>
<action application="record_session" data="$${recordings_dir}/${uuid}.wav"/>
<action application="sched_hangup" data="+30"/>
<action application="playback" data="${hold_music}"/>
</condition>
</extension>
Dial 732664 (i.e. "REC MOH" :) and just wait. You'll hear music and then at 30 seconds it will stop recording.
Happy FreeSWITCHing!