Would imagine the aircraft would be on standby 24/7 365. Don't think there can be "regular" hours in this line of work i.e. you can't expect the peoples of the Scotttish isles to have ruptured appendixes (appendicies) between 9am and 5.30pm Monday to Friday.
Flying about the isles I have seen the Scottish Air Ambulances at various fields, the heli was on Islay on a Tuesday morning and the King was at Wick on a Sunday afternoon when I have flown in. Sorry cant help you anymore with the rosters but would imagine most of it would be spent on quick response stanby.