Under usual circumstances, you should expect to spend between 10 and 20 days a year on your governing responsibilities. The top end of this commitment, which equates to about half a day per week in term time, is most relevant to the chair and others with key roles, such as chairs of committees.
Initially, we would expect your commitment to be nearer to 10 days a year. However, there may be periods when the time commitment may increase, for example when recruiting a headteacher. Some longstanding governors may tell you that they spend far more time than this on school business, however, it is fairly common for governors to undertake additional volunteering roles over and above governance.