There is no exact word, every tyre is different. Some road tyres will benefit from preheating, but don't overbake them as they can overheat on the track. A heavy bike, ridden hard on the track, will soon overheat the tyres, which can be seen in the tread.
Race tyres can range from 60-90 degrees. But we don't all have tyre temperature guns, so we usually use the touch method. Now, this is where most people make mistakes. The surface of the tyre may be warm, but the carcass and wheels may still be cold. Don't just put your hand on the tyre to feel the actual wheel: is the tyre warm?
In perfect conditions, the wheel should be warm and the tyre surface almost hot enough to touch - depending on the tyre. On a professional level it is important to control the tyre temperature, but time is not an issue. If the heating cycle is within the temperature specified range, around 80-90 degrees, there is no particular limit as far as time is concerned.
Most importantly, once a racing tyre has been preheated, you must be able to keep that heat in the tyre. If you can't keep the heat out of your tyres then try a different tyre such as a track tyre with a soft compound tread, or a sports road tyre with a lower operating temperature range.
"The minimum temperature depends on many things," says Salvo Pennisi." Track temperature is important because it affects the build-up of heat. If the compound does not reach the proper operating range of 80-90 degrees, then many problems such as tearing and unforeseen slippage can occur. The heat build-up of the compound is also influenced by asphalt roughness, lap times, bike set-up, rider style and many other factors.
Tyre warmers are also important when the weather is warm. In the height of summer, the sun alone is not enough to bring race slicks to the correct temperature (remember the rule: the wheels must be warm and the rubber almost too hot to grip). Even on Spanish circuits, use tyre preheaters to bring them up to the correct recommended temperature.
An often forgotten tip is to put the tyres back on the warmers as soon as you get back to the pits, as race slicks don't like to cool down and then warm up again and again, whereas some dedicated race slicks only have a permissible heat cycle, going from cold to hot and back down again.
Once your racing tyres have reached the correct temperature, keep the tyre warmer on at lunchtime to maintain this temperature but do not overheat. Most tyre heaters have a thermostat or temperature gauge to control the temperature. Also, at the end of the day, simply place the tyre warmer on the tyres, but turn it off. Allow the slick tyres to cool gradually for better performance and longevity.