Once you can hold them flat, you wont be hyper extending them when you are Breakdancing. Footwork, handstands, flares freezes all require that you can hold your wrist at least flat on the ground with ease.
Flexibility is half the battle, now you need wrist strength to hold your body weight on one wrist and bounce, thats about the most extreme case. You wont be able to jackhammer unless your wrists can handle that. Here is one exercise you can do at home.
You make it harder the closer it resembles a push up. The other exercise which works best is practicing a false grip and doing pullups, you need rings for this though.
Once your wrists are strong and flexible you will be able to practice all the moves without pain. So getting them prepared needs to be your first goal in breakdancing.
My wrists already hurt, what do I do?
Overuse is common, the way to fix is to not do things that make it hurt. It has to heal then you can train them again. Failure to follow this will lead to more pain and strain on your wrists.
How long before they are ready for Breakdancing?
You can get them flexible in about 1-2 months. The strength portion takes more time. It all depends how much you are training them though. Wrist strength and flexibility should be part of your normal warm-up before you breakdance until you stop experiencing any pain.