Tough call. Either you buy now or wait and risk not being able to book a decent ticket.

If you buy now or before he/she starts, I would say base the return date on whenever the last final is scheduled. I assume this is U of M your kid is attending? Fall finals can be found here: http://www.umich.edu/~regoff/exams/fall.html. Spring 2005 doesn't appear to be posted.

Worst case scenerio is your kid may be done a few days earlier then when all finals end.

On the other hand, you could wait and see if his/her schedule firms up a bit more. This can be difficult to do sometimes as class schedules are always in flux and for a week or two each semester, as students add and drop classes. By waiting, moreover, you also run the risk of not having a wider availability of flights/dates and could end up with a more expensive ticket.

Let me just say I can empathize. In grad school, it was a huge problem for me. First, I was flying to Florida (a popular place over Thanksgiving and Winter). Not to mention, I had self-scheduled exams which made it more difficult to assess when I would be done.