My dentist also just told me it's ok to leave temporary cement in with my permanent crown? How long does temporary cement last?

I use temporary cement when I am concerned about the root of a tooth not being healthy but it is not clear and would like to give the tooth a chance to heal.  We also use temporary cement for Implant Crowns-I have had temporary cement in crowns for many, many years.  Cement really is only used to keep bacteria out of the space that is created when a crown is made-There are new cements on the market that 'bond' a crown to a tooth but if the crown is properly made, the glue really does not matter.  If a temporary crown comes out, it is just recemented.  There is literature that shows that temporary cement eventually under goes a chemical change and holds like permanent.  Thanks for asking-Good question!!

Sometimes a tooth or crown is just not ready for the permanent crown to be permanently cemented.

The cement inside can last from 3 to 18 months. There is nothing wrong with this approach. 

My concern is that there seems to be a communication breakdown. You may want to re-contact your dentist to get a more clear answer of why he's approaching it this way. 

At some point, either the crown will just come loose and come off, or he will want to remove it and finally put the permanent cement in.

You job is to be aware of any change in that tooth. If it starts to feel funny, or you are getting a new taste from it, that could mean the temporary cement is washing out. Also beware of any sense of the slightest moving of the crown. That's the only warning you might get that it is ready to fall off.

Stay in touch with your dentist, he should be able to help you manage this well.

Temporary cement in a permanent crown is intended as a trial period to ensure the crown will work well for you, and that the tooth below will not need any further treatment.  Sometimes temporary cement holds so well, that the dentist is afraid to force off the crown for fear of breaking it.  If that is the case, you can probably help your nervous dentist out by buying a box of Milk Duds and letting the sticky candy safely pull the crown off so that the dentist can replace the cement.