A very quick word of warning to the above posters: the modern glass cockpit turboprop has a vastly different deicing boot design to the older types.
The older type's design meant that it was essential that a build up of ice was allowed before operation to prevent 'bridging' which most certainly does occur and can render boots totally ineffective. The modern design has interlapped elements in the inflation cycle which breaks ice off irrespective of build up. It is this latter design that is best switched on as soon as ice is detected.
Beware of confusing the two designs and techniques. It can be, indeed has been, fatal.