If you are looking to replace a crown and you take the proper antibiotics ahead of each appointment and you have been given medical clearance from your doctor, then I would not be concerned with having the work done at all. In most offices theses days there are two appointments necessary for having crowns done. The first one is for removing the old crown and preparing the the tooth for a new crown. And the second one is for delivering the crown. Your dentist is right to say that it can 3 weeks or so to get the crown back. Temporary crowns are made of bio-compatible acrylic material and if done and places properly have very minimal risk involved with them. Wearing the temp gives the lab technician time to hand-craft your specific crown to fit your basic needs. This is how my office does it as well, but our turn around time is only 2 weeks. This time can vary based on the lab used, the location of the lab, the business of the the lab and special considerations or materials your dentist may have for that tooth. All this being said there a dentists around that can do a single crown in one day. If you are that uncomfortable waiting I am sure you can find a dentist with a Cerec or E4D which is a specialized machine that can allow the dentist to prepare and seat a crown all in the same appointment, most like a few hours long, but done in the same day. I don't think this is a must so if you trust your dentist and are ok with wait I would stick it out.