My approach with Birkenstocks was the same - I was going to give them a two week trial. The experience, however, was just the opposite - everyday the shoes felt alittle more comfortable and with continued use, my feet did get better. After 3 mths - almost no pain - and now I'm at the completely pain free point after an entire year of wearing them.
My experience says ... Let YOUR FEET tell you what works best for you.
P.S. Glucosamine - gave it two weeks and felt it also was a big help. Still on it once a day (total 500 mg).