The good thing about smaller programs is that it doesn't take long to get to know all 10 residents (in the case of AGH & Union). I spent a random week at Hamot, knew everyone in the program by Wednesday, and ended up getting an interview there. If you are having a difficult decision b/t AGH and Union, I would definitely recommend doing 2 weeks at each. You will most likely get interviews at both.
A month at those programs may be overkill. Odds are that as long as you work hard during those two weeks, you would be looked upon in the same light as a guy who spent a month there. Two weeks at a small program is enough for them to get an idea of who you are and they will definitely remember you when interviews come around. And although the official applications say you need to spend a month, if you call up the program secretary in charge of rotations, they are usually really nice (I interviewed at AGH, and I can personally vouch for the coordinator up there) and will work with whatever dates you have available. Just give them a call.
With a bigger program like Maryland and OSU, then you should spend the entire month. One of my buddies just matched at OSU and is thrilled. He thinks the new chairman and the residents are great. I would do a whole month at UofM or OSU and then 2 weeks each at AGH and Union.
Finally, most programs only let you have 3 LORs, but some require 4, and some require a non-ortho one (Tufts, for example).
Hope this helps.
