I came across freezing rain just once, flying into Bournemouth, through some TCUs. It was raining but the OAT gauge was showing -2C so I immediately descended a couple of thousand feet (told ATC and got on with it, mostly OCAS) and by the time I looked outside again, in VMC, the OAT was about +1 and I could not see any ice sticking.
Re structural icing in clear air, I have never had this but have had some kind of effect on the IAS i.e. the pitot tube. That's why one should have pitot heat on anytime below about +5.
Structural icing in IMC flight is a big subject. My take is that you will pick up ice eventually anytime the OAT is between 0 and say -10C (in non-convective cloud) and down to a lot colder in other types of cloud. But it is very random; you could fly for 30 mins in IMC and get no ice at all and then Bang and you get 5mm within seconds.