A month. One should be off the pill a month before trying to conceive. It is also important to be supplementing Folic Acid for several months prior to conception. Finally, it's wise to see a gyn for preconception counselling before trying in order to best assure the healthiest odds of pregnancy.
When ready. Stop the pill whenever you are ready to try to get pregnant. Some women will conceive the next month right away and others may take longer.