How long does it take for crown hair to grow after hair transplant?

Full growth takes a year. The crown is notoriously difficult to get good coverage. You should expect stll to see the scalp skin even after all the hair has grown in,Dr. Robert Jones

In usual transplants. the growth starts appearing in 5-6 months and the full results can take around 9-12 months. In my opinion the crown growth takes longer, somewhere around 12-14 months. Also crown growth as compared to frontal do not give that much coverage because of anatomical variation. 
I would suggest you to wait for another 6 months and do discuss your expectation with your treating surgeon. 

Hairs start to grow from 3-6 months and one can expect about 80% of the final result at around 6 months. Not all patients are the same and this can vary. In your case, I would wait the final 12 month period before making any judgements. It's likely that the transplanted hairs are in different phases of their lifecycle.