How do you know the exact amount of time needed to fix each bug?
If I fix a bug, I don't know if it'll take 5 minutes or 5 days. I only know that the moment I actually fixed it.
Discussion
He Wills them fixed.
Thats not the point of timeblocking. Its to add a sequential order of things to do in your calendar so you have a visual representation of your day so you stay on track. If its a big feature ill make rough guesses and make the timeblock bigger, while bugs i will make 15 minute to an hour timeblocks. Its fine if its not accurate, this is why i created viscal, i can shift whole blocks around and reorder tasks if things take too long. I eventually move the tasks to the next day if i don’t get to it. 
having a plan is better than nothing.