This is one of those 'how long is a piece of string' questions. It depends partly on what breed you keep, the quality of your pasture, whether you set some aside in Spring to make your own hay, the weather in any particular year, whether you want the grass to be grazeable in the winter as well as the summer, etc..... Most people have a topper so they can cut the spring growth back to a reasonable length before it forms flower stalks.