has anyone grown pak choi?

The mixed salad I sowed has pak choi in it and it seems to be doing quite well. I have bought some seeds of a green variety as it sounded great (Green Revolution F1)
How long does it take to make the hearted up stem base, please? The salad version is romping away but it was started under a cloche back at the start of April and as yet there are no real hearts.

Am I just too impatient?

Thank you Mumofstig ---  they have been in since 2nd April, so I thnk this variety must take a long time. Don't know what it is as it was in a mix. Maybe the green ones will be quicker growing 

I've have never done this salad thingie, but might it not just be possible that the variety in there won't grow much of a heart at all?

I grew a row last year I put mine in after my early potatoes.It was ready about october.

                                                                                     celery









                   

Do you not find that early sowings have a tendency to bolt? (I've been delaying sowing until end of May ...)

Thanks everyone - I shall be sowing some this week and then some alongside the leeks in the space left by ealry spuds in a few weeks' time -- good tip re cabbage white caterpillars Kittiwake. I shall have to use some of the netting left over from the brassicas bed I reckon.
No sign of bolting yet from those I sowed ealry April --- just loads of yummy leaves