I'm wondering how long in time should it be from having a root canal done to having a metal post fitted and then a new crown fitted to the post?

Prognosis?: After root canal treatment (rct) is done, the final treatment depends on the likelihood of survival for a long-term. Generally, it is best to do all right away: the tooth must be sealed well from the root tip (done by doing rct) ; the top, which requires fill/crown etc. The tooth after rct may need to be monitored sometimes. Since this depends on your condition, ask your doctor for specifics.

1-2 weeks : Follow the recommendations of the endodontist that did your root canal. Usually this is done within a week or two after you have healed from the root canal.