Agreeing to be on someone's thesis committee is easy. One day someone drops by your office with either a written or oral summary of the project and an approximate time line. If the work sounds vaguely interesting or you owe the PI for serving on your student's committee you agree and move on. There is an occasional meeting obligation here and there, but overall it's not a big deal. Until the end.
I've actually made a concerted effort to try and space out the potential end dates of the students whose committees I have agreed to, but somehow it always seems that I go through stretches where I will have 2-4 defenses all within a week or two. Obviously, these dates are constrained around the semester deadlines for the grad school, but one person runs a semester or two long, another goes shorter than planned and they all cluster together.
Given that defenses appear to be attracted to small calendar windows like moths to flame, I think I need to rethink the number of committees I agree to. I've honestly lost count of the total I am on, but I believe it is somewhere around 10 (though three defenses in the last two weeks have thinned the herd). So how many is ideal? What is the balance between MS and PhD committees? I can't tell whether I'm over committee committed or just suffering through a concentrated stretch of obligations.