maybe this chart will help you.
http://www.sewing.org/files/guidelin...dle_charts.pdf
or this one
http://www.joann.com/on/demandware.s...df/PA00223.pdf
here's another one:
http://www.sewmamasew.com/2009/06/se...o-choose-them/ or this one:
http://www.craftstylish.com/item/145...ge-your-needle
. I usually change my needle when I see a difference in the sewing. If I have gone thru many thicki layers for a long time ( working on a quilt) it will be one needle per quilt. If it's just sewing on plain fabric, it may not be as often.