There are two reasons for not having them done all at once. The first is that if they are in more than two quadrants, it will mean having a huge amount of novacaine received in a single day. The other reason is that when the amalgams are drilled out mercury is released and some is absorbed into your body. It takes a few weeks after for the mercury level in your blood to drop back to where it was before the removal. Having half done then the other half done a month later would give your body a chance to get the mercury level back down before more are done. While having all 5 done at once is possible, if I were in your place I would choose to have it done in two appointments, spaced a month apart. Keeping your mouth open for over two hours is also quite difficult. I don't recommend it unless it is absolutely necessary.