'enry,
At cadwell we'll have 20 minute sessions (probably about 18 mins in reality), and it's only ever taken me a lap and an arf to get the tyres as hot as they're going be... that's about 3 minutes. No warmers. Depends what rubber you're using of corse!
Weaving about on the track doesn't tend to warm tyres very quickly... when the pro's do it they're probably trying to scrub off releasing agent rather than warm they rubber. Feeding power thru the back tyre, and fairly heavy on the front brake will convert energy to heat. Or just blag some tyre warmers!
Just my opinion of corse. Most important thing to remember is cakes and wine
[Edited on 28-2-2005 by NBs996]