Usually you work in multiples of 10. Or if you want a faster decrease you could work in multiples of 7.
For 10, you'd work the first decrease row as k8, k2tog all the way around, and then knit a row even. On the next dec you'd work k7, k2tog, then k6, k2tog, etc. (with an even row between) until you get down to a few stitches that you'd pull your yarn through to close the top.
For multiple of 7, you'd start with k5, k2tog, k4, k2tog, etc., with an even row between.