There are many factors that affect tire life. The track surface itself, certainly your setup; shocks, springs, roll bars, and tire pressure, the air and track temperature, as well as your driving skill/technique are a few.
Generally, as tires age they get harder. This is more of an issue for street tires since they tend to be used for much longer periods of time.
All things being equal, your best qualifying lap will be made with a fresh set of stickers (brand new) or scuffed (one good heat cycle - no abuse). The scuffed tires seem to get where you want them to be quicker and are more consistant than the stickers. You can either take a lap to bring them up to temperature, or increase the tire pressure slightly and go for it on the first lap. I prefer to warm everything up and then do 2 hot laps.
I like to use old tires for practice and go to fresh rubber for qualifying and racing. Most times the Hoosiers get pretty slippery after about 8 - 10 heat cycles, but I can learn a lot about a track from that experience. Usually when I go to fresh rubber I can almost drive a different line and can drive 1 - 1.5 seconds faster. On several occaisions the used tires seem to get better and better no matter how many heat cycles I put on them. I have sometimes gone to stickers and gained almost nothing. I can't really explain why except that all the various factors sometimes must be able to work together to prolong the life of the tire.
The bottom line is You should know when the tires are done. Check carefully for wear after every session (cords showing, flatspots that you can feel while driving) and see how they feel to run. Don't risk yourself or your car on a set that you can't control, but don't throw them out unless they deserve it. I don't really think there is a hard and fast rule - too many factors involved.
(These are my opinions based on my own experience, and some may not agree, but I hope it helps)
Also, I stand corrected, Janni is right as usual. The red neon is best for the full slick - my mistake... Try the Hoosierdaddy brand neon lightstick for best results.