From koffienl

Create a timer device:

      "resolution": 60,
      "xLink": "",
      "id": "daytimer",
      "name": "DayTimer",
      "class": "Timer"

A resolution of 60 means you want to update every 60 seconds (think of your DB!).
You can start and stop the timer in rules, and use the count:

IF lamp is off THEN stop daytimer
IF time of daytimer >= 300 THEN log "timer is 5 minutes!" and stop daytimer and reset daytimer