How long does it usually take to defrag the boot drive?
It depends- How big is the drive, how fragmented is it, how fast is your computer, are you defragging files or consolidating the disc, etc.
Defraging files is the quicker of the two.
Consolidating a large drive (1 TB or above) will take a long time.
Confusingly Consolidating is also usually referred to as defragmentinging, so you have to know which of the two you are actually doing.
See this for more info of the different "Defrag" types - Defrag or Consolidate, Defraggler can do either or both depending on how you use it: