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

You can expect new hair growth usually within five to six months following the procedure. Moreover this is also the stage where you will start observing that the new transplanted hair has thickened considerably. By 6 to 10 months, you will be able to notice the most impressive difference since the initial procedure. After a full year, you can expect to see the final results of the procedure
The new hair follicles may not necessarily grow evenly, but it will all grow eventually. The new transplanted hair will start to grow thin but will gradually become denser over time.

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. 

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

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.

First results You will be able to see after 3-5 months. Is the same time for front part as well.

Hair grafting with Follicular unit extraction is a very reliable procedure and has excellent results. The time period for hair growth is variable depending on your age amount of grafts and follow up treatment(ie biotin, Platelet rich plasma, laser cap) you are completing. Most patients should start seeing results within 6 months with continued growth up to a year.  The crown area is probably the most reliable and stable areas after hair grafting.

The crown will grow at the same rate as the frontal hair transplants. This is usually between 2-5 months when it will break through the skin and 8 months to styling length

No matter where the hair was transplanted it can take up to one year to see the final results. In some cases patients even see growth up to 18 mo. If it’s only been 6 mo. you will see more density over the coming months.