The latter, scramble time for the Sea King SAR helos are 15 mins during the daytime, and 60 at night. (Will reduce to 15 with a rule change shortly).
Usually, the RCC will scramble a helo rather early in the game to be on the safe side, let's say within 10 minutes after +30. (Get the crew to the helo, and when they are ready, they might be able to give a more narrow search area)
I dont't think the RCC even have the right to scrable before +30 minutes anyway, since this is buffer to allow for headwinds bad weather etc.
And as detection goes, the searches i have participated in /initiated we usually narrow the field a lot before we hit +30min. (Allert all parties, CRC's, operators, aero clubs, AF
Sqn.) We don't have to, but a few heads up calls have given great results. (Eliminate all the known sources for the aircraft beeing late)
But I have to agree that obtaining a flight following or FIS service does give us a much greater chance of finding a aircraft in distress. This is really useful in areas with poor/no radar caverage in lower IFR altitudes, which we have a lot of where I work (mountainous terrain)