How many days after teeth whitening should I get my dentist to colour match a new crown? (Photo)

We usually wait a week or two for the color to stabilize prior to selecting a shade for the new crown. 

Safely wait 10 days to 2 weeks before having a shading taken to match the new whiter shade of your adjacent teeth.  Be aware that porcelain can not be further whitened once selected...so you must maintain the whiter brighter shading on a regular basis.  ( perhaps one week-end a month with your whitening trays)

I usually wait 2 weeks for the color to settle. If the teeth were much darker than the shade you are lightening to, you may want to wait even a little longer as you will tend to get more rebound. Keep up your whitening once a week or so to maintain where you are at after the crown is in