You choose the deferred period by asking yourself if you were off work sick how long would you be able to maintain your lifestyle before you start needing financial help. If your work pays you full sick pay for 3 months, then you go to statutory sick pay after that then it would be a clever idea to have a 3-month deferred period. If you are employed or self-employed and do not get any income at all and do not have any savings it would be a wise idea to have a policy which pays out from day one.