Depends what I'm doing. Sometimes an 80/12 (or whatever) is needed. I have a sheet that I mark down which needle is changed to what.(eg: Needle 9 - 80/12 S) Generally, if you get three thread breaks on the same needle, it needs to be changed.
I also change them with the semi annual all over check up(making sure that there are no hidden problems you don't always see with reg. oiling and maintenance)
One thing you might want to do as well, the colors you use most ( eg:black, white, red, ) change them around all the time. Don't keep them on the same needle all the time. That way all the needles get used roughly the same amount.