Bleeched buzzed and toned how often

I recently bleached my my short hair and toned white which was buzzed on the sides to a #1 and about 2 inches on top after a 2nd bleaching a week later the longer 2 inch hair on top broke in half to about an inch. No problem buzzed the top with a number 2 guard sides with a number 1. It's a week later and the dark roots making my head looking 2 toned. When and how often can I bleech or tone to keep the white buzz cut looking good? Should head be buzzed prior to new bleaching ?