when will my cabbage be ready?

Hi,  I planted some winter cabbage last year. They are now very big but still no hearts. Cooked some the other day, and it was quite bitter? The question is , are they too late or too early?

Winter cabbages are to be harvested over the winter.
Leaving too long will turn them bitter as they have gone to seed.
What variety were they?

If leafy greens grow very slowly, they can get bitter with time.  I would be tempted to call it a day and start using these up before they get too bitter to eat.

You might find blanching them quickly, then draining the water and finishing the cooking with fresh water makes them more palatable 

If they were Durham early, you can pick like spring greens in March or as a conical or pointed shaped heart in April/May.
You would sow Durham early in July or August and plant out in September/October.

When you cook your cabbage and to reduce some of the bitterness, try this;
Do not omit the salt, it actually helps with reducing the bitterness.
You could also add a small amount of sugar, or cook with apple, peppers or shredded carrot.
If you make coleslaw you always add sugar as the cabbage left can turn bitter when made as coleslaw.