As Erin said, it is not a word we use a lot in daily conversation. The weatherman might say, "It's going to be a scorcher today!", meaning that it will be very hot. As far as the correct temperature to use this word, I think that is probably more relative depending on the temperatures you are used to. For example, it was 36 celsius here today, but it wasn't scorching by any means, just hot. In late July and August when it is 43 celsius, I would consider it scorching. Typically we just say, "It's SO hot today!", or "I can't believe it's SO hot!". Someone in Canada, however, might feel that it is scorching at 24 celsius, while I would be comfortable or even a little chilly if it was windy. Humidity would also play a big factor as well. Hope this helps!

This word is quite common in the UK. It would not be unusual to hear or read the word scorching. - 9 month old girl dies after being left in scorching hot truck - Scorching hot ash for Guatamala volcano... - Scorching hot playground equipment can cause burns - UK weather forecast: UK to be hit by scorching hot weather system this weekend - Scorching hot days and intense thunderstorms incoming for the first half of April

It's not a word I would normally use, and I don't usually hear others use it either. However, you could use it when it is extremely hot outside.