Broccoli Regeneration

Hi Everybody,
I am on my second crop of broccoli, and have discovered that they keep growing after the first, main, head has been cut. Even though it is winter in this part of the world, they just keep going, and producing small heads (which are quite flavourful.)
So, how long should I expect them to live? I even had one that I had cut down to about 6-inches (planning to just till in later), and it is getting lots of new leaves, and a new head.
If they can survive winter here, should I just keep them permanently, and keep harvesting the small heads? Or will finish at some time?
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