I would imagine it is when the met office report that they exist. And bear in mind that the FAA regard "forecast icing conditions" as "known icing conditions".
You could also infer it from the flight manual from when the anti-icing needs to be on - e.g. +4.4C for the 206, but Robinson have also issued warnings for +30 in humid conditions, so my own inclination to take the one the jury would take notice of!
Phil