Timer to turn off

is it possible to turn the LEDs on through an infrared sensor (pir) and turn them off 30 seconds later if there are no further signals from the PIR?

yes.
one possibility is button with type PIR or PIR sensor switch usermod.

Good, where to config time to turn off ?

either on PIR sensor (SR501) or in PIR sensor usermod (settings page)

Thanks, in the next days I check.