There’s no qualifying benchmark that indicates you’ve chain healed your way into the pros of healing. How do you know, then, when you’ve got what it takes to raid?
Generally if you’re successfully and easily healing heroics then you are ready to raid. Of course healing raids isn’t as easy as being a heroic farmer; you have to have the skill to react to emergency situations and the situational awareness to stay alive yourself.
Cata heroics are a big step up from Lich King content and serve to teach a lot of raid skills, especially such important principles as don’t stand in the fire. These principles of raiding will come in time with practice.
Another important facet of healing that is folded into being able to heal heroics is smart stat gearing. If you don’t know your class well enough and don’t prioritize the correct stats you can easily fall short of making ends meet. Gemming and enchanting are another important resource to consider that can add some extra breathing room to fights like Admiral Ripsnarl. A good practice is to gem and enchant all 346+ gear and heroic gear as you’ll have those for your first couple weeks of raiding.
Overall you’ll know that you’re ready to heal when you feel that your gear is well put together and that you know your class well enough to comfortably heal hard encounters.