If you are breaking a foot in that amount of time you need to be sending them back for warranty. There is no reason for a carbon foot to break more than every 3-5 years IF that, under lots of use. I would suspect that if you are breaking feet that quickly you may want to confer with your prosthesist about a heavier category foot. Cycling does nothing more to stress it than walking or any other activity, it is just seeing stress cycles for which it is designed to take into the millions.
The only foot I have delaminated was a Highlander, and it was honestly right AT my weight category and used harshly which still lasted almost three years. The best carbon foot I have had so far was a Renegade. Stiff but tough. I am on a Rush Ability right now. I haven't used it long enough to know how rugged it will be, but after getting used to not being on carbon it is by far the most natural acting foot I have used thus far.