Every six months, twice a year. No bitch should be bred from until at least her second season, or preferably over 2 years of age.
As kita says, this is not something to be taken lightly, and cockers particularly suffer from several health problems that need to be tested for.
I would also say you need to do a lot more homework before even considering breeding, as knowing when a bitch has seasons is one of the most basic requirements, and although it might seem like a good idea, it is definitely not a money making game and can be heartbreaking - what would you do if you lost your bitch while whelping?
If you want another one, why not buy one in, and save yourself all the grief that goes with it.