Timeouts on cookie persistence profiles on LTM 10.2.1

Running LTM 3900, software 10.2.1. If I create a persistence cookie from the GUI, the config stanza for it looks like this:

profile persist mytest {
   defaults from cookie
   mode cookie
   cookie mode insert
   cookie name "OHHAI-IAMYOURCOOKIE"
   cookie expiration immediate
   override connection limit disable
}

I don't see a way to specify or modify the timeout from the GUI. If I change the timeout using tmsh, I see the change in my text config but there's no change in the GUI config display.

profile persist mytest {
   defaults from cookie
   mode cookie
   timeout 180
   cookie mode insert
   cookie name "OHHAI-IAMYOURCOOKIE"
   cookie expiration immediate
   override connection limit disable
}

Is this a tmsh-only configuration option? There also doesn't seem to be a way to remove the explicit timeout from the profile config. If I try to remove it using tmsh, I get an error message 'Syntax Error: "timeout" read-only property'. Is there a way to do this?