I tend to use thinner bore drip tips, so I have only ever mostly encountered minor spitback-a little annoying, but never enough to burn.
I DID burn my mouth slightly once when I was getting into TC, using TI, and I was happily vaping away, then WHOOSH, it set fire, only for a split second though.
Still not 100% what caused that, but I think the coil may of touched some metal inside the tank, as there was not much space in there, I learnt my lesson though from that one regarding room for coils.
My el-cabron and twisted messes with wide bore do give more spitback, and I take deep inhales, but it has never lasted for few days. That must hurt