I did:
one 4-week externship (LSU NO)
two 3-week externships (Minnesota & Cook County)
one 2-week externship (Parkland)
At LSU NO: had a blast, by the end of 3 weeks, I believe I've gained the most that I can gain at that time of my dental knowledge. By the 4th week, I started to let the other newer externs to do everything and I just stand around and chatted with the new interns/residents. Looking back, 4 weeks was too long, but what a great first externship for me!
At Minnesota and Cook County: had a blast, to me, I strongly believe 3-week externships were perfect. It takes you about a week to know the residency system and weekly schedules. By week two, you'll most likely know where most of the things are in the hospital (paperwork, different wards, underground cafeterias), know most if not all the attendings, staff and residents' names, and getting really cranking on the outpatient clinic. By the third week, this is when you hopefully have earned the full trust of the residents where they may allow you to do some cool stuff and let you do more things on your own.
At Parkland: had a blast, residents there were hilarious (great sense of humor). Now that Doggie's there, he definitely had decreased the "coolness" for those guys. I felt that 2 weeks were fine, but I was sad that I had to leave when I was just getting to know the guys and having a blast! Like I said above, it takes about two weeks to get into full swing of things in every aspect and by leaving was kind of a bummer. I initially had a 4-week externship approved at Parkland, but I had to cut my participation length down due to lack of financial means (damn financial aid didn't disburse in time).
Bottom line, here are things I highly recommend in regards to externships:
1) do 3-week externship(s) at place(s) that you are really interested (it will give you chances to really know the program)
2) do 2-week externship(s) at place(s) when you are running out of time to do externships (your 4th year of dental school) and/or you want to see as many programs as you can (you'll miss a lot by leaving the program only after two weeks)
3) 1-week externships to me is just a waste of time - no good
4) do a 4-week externship only at programs where they allow you to do a lot of hands-on stuff where you can really experience what an OMFS residency is like AND you're starting early (if your first externship is b/n your 2nd & 3rd year) OR if it's your first externship ever
BTW, if you're doing an externship, I don't care what the participation length is, please take some calls! And when you take call, stay in house! Your externship will not be a true experience if you don't take calls frequently and if you don't stay in-house! The resident that you take call with, you will really bond with them. Most of the time, you can choose and pick which residents that you want to take calls with. I've met many externs who chose not to take calls or if they did take call, they didn't want to stay in-house.