Not sure why folks not marking SOLVED.. fix is simply as stated:
after edit to luci file, run: /etc/init.d/rpcd restart
reconnect on the GUI and
ubus call session list | grep timeout
will show the new timeout.
Because is generally considered rude [for someone with the authority] to mark a necropost as solved - on a whim that the Orginal Poster is now satisfied with the same solution.
Especially given the OP said themself that it didn't solve their problem.
Also BTW, there's been version changes in the web GUI since your post 4 years later.
right,
a) So sorry, only until hit REPLY noticed "4 years later", felt like in a Time Machine, what? how? then figured it's an old thread, sorry..
b) Totally understand about rude/towards OP, but maybe there should be another "Knob" for ANYONE to mark "acceptable solution", possibly several 
not just the OP, more of a "decenteralized approach"..
The moment OP said it didn't work, they didn't re-login to verify timeout took effect.. all set, hopefully future readers will figure better.