I used to block off every 9AM-10AM, 4PM-5PM and 1PM-2PM slot as busy slots. If someone really needed to have a meeting at one of those times they could ask me. Because often, those were the only slots the 10 people in the meeting all had open in the immediate future so people would then schedule meetings at those times. I grew tired of 4PM-5PM meetings which extended to 5:30PM, 6PM etc.