I use mine for work more than not. Because I do such a variety of jobs (and even my own sewing) I am likely to have jeans, ITY, silk knit, linens and everything else, I change them often.
I tend to group them together so I use more time on a needle. Work wise I do no more then 4 garments on one set. Personally, I do about 6 or 7 before changing. I use either 12, 14 or 16 size needles. My overlock, cover stitch (Brother) and main hem machine/back up (201) use the same needles and I buy them by the hundreds in sharps and BP. I buy either 100 needles at a time or 500.