Hi Melissa,
If the doctor told you to wear it for ab work then go ahead, but if you've been doing cardio for six days a week for a year, after the c-section, and are not feeling any pain, I would ask before I put it on. The kickboxing, I'd ask about too before I start, as I can see arguments for and against wearing a brace during that and it's better to check. And if you're unsure about wearing a brace to began with, get a second opinion, you are the one in your body, you know it best. If you feel this isn't the best advice or your not sure about the advice, find someone who can give you another opinion that you trust. Only takes a little bit of time and you'll be sure, or have questions to ask doc #1 so you know what, why, and how, their treatment is suppose to work and then you can decide which doc to go with.
Also there is a rule to braces, they help the muscle do it's work. Helping means the muscle isn't doing all the work itself (good, if the muscle has been injured or has had surgery). In turn it is much harder to strength a muscle that is in a brace, as even just the standard set of exercises it will not have the stress it would without the brace. And if you wear a brace when you don't need it, you will lose muscle strength, if you do it often. If the muscle only works half as hard in the brace, the normal stress that it is use to taking is less (say like you doing cardio without the brace and now doing with the brace, stress on abs/back will be less), and if you don't put that stress back on it after a certain amount of time, it loses what you built up. You won't be completely weak, but whatever help, the brace does will subtract from your current strength in the abs, and maybe even in the back. Time is different for everyone; some people can go a couple of weeks and not lose strength some people lose strength in a couple of days.
I'd also ask the doctor to give you some strengthening exercises for both the back and the abs, to do without the brace. Or with it, if you're really still that weak from the c-section. This you'll be sure to get stronger and get rid of that brace faster, and be able to do whatever workout or exercise you want without worry.
Good luck,
Kit