It depends on what you are doing,the tip size and material thickness.The best advice I can give you is read the manual that came with your equiptment.

http://www.adtdl.army.mil/cgi-bin/atdl.dll/tc/9-237/toc.htm
Look here- this is the army welding manual.

Basically when you light the torch before you turn on any oxygen the flame will be yellow (you should have the gas valve open enough so that most of the smoke is gone). Then as you open the oxygen valve the flame will first brighten to white, then proceed to a state where there is a blue cone near the tip then a whiter cone and the whole surrounded by more blue. If you leave some of the white, usually not more than again the length of the inner blue, your flame is carburizing or reducing. The terms are interchangeable. Add oxygen till the white is just gone and the flame is neutral. More oxygen than that makes the flame oxidizing. Generally neutral to slightly reducing is most useful. The instructions that you may have to ask for when you buy the filler will be a helpful guide.

Have fun-
Bill