How long should I wait until I get the crown for my root canal?

You should get this taken care of as soon as possible. In my practice, at the follow up appointment after a root canal I usually replace the temporary filling with a bonded composite filling or crown build up as needed. The tooth can also be prepared for a crown at the same appointment. The quicker a crown is done, the better the odds of avoiding some sort of fracture in the tooth.

Try and pop into the dentist so they can re-place the temporary filling - its should not take long to do normally. The dentist  then needs to the re asses the tooth to see if the root filling was successful . They will need to decide if they plan to place a filling or a crown  next. Assuming they plan a crown and the root filling has been successful  the dentist will organise an appointment to prepare a post crown or core and crown which will involve a laboratory making your crown and would take a couple of further appointments.
Once the crown is fitted it should  look and feel much more comfortable  and help prevent the tooth fracturing.