if you go on to the csa calculator on the csa website and imput your earnings and when you have your child they will tell you what you should be paying.
I should imagine you wouldn't have to pay anything if you have them for half the time, so that would suggest aound 180 days per year. I recently emailed them to ask if my partner had to pay as he was taking the children away on holiday, so why should he have to pay the mother while they weren't with her? The CSA response was that there is only a discount on the amount of maintenence paid if you have them for more than 52 days of the year.