If it works like on other Fiats, You need to pull the light stalk towards You (like flashing) before You leave the car (engine off).
On my old Bravo, Stilo and Punto each pull added 30 sec. to the time, but that may be moved to the settings menu.
I was partly right. Read below
It's described in the manual on page 24:
HEADLIGHTS OFF TIMER
(Follow me home)
Activation
With headlights on, bring the ignition
device to STOP.
Within 2 minutes, rotate the light switch
nut to O: the timing set previously
through the Menu (0, 30, 60, up to 90
seconds) starts.
Deactivation
This function is deactivated by
switching on the headlights, the side
lights or bringing the ignition device to
MAR.
"Follow me home" function
interaction with system
UconnectTM system
If on the menu Uconnect™ the timer is
set to 0 seconds it is possible, using
the high beam lever, to turn on the
lights for a pre-set time of 30 seconds.
It is possible to activate the lights 7
times up to a maximum of 210
seconds.
If, however, a timing (of 30 or 60
seconds, for example) has been set
through the "Settings" menu of
Uconnect™, each time the engine is
switched off, the "Follow me home"
function will be activated for the
duration of that timing (you will not be
able to change it using the light switch),
only if the ring nut A fig. 29 is rotated
to position O.