You have to be out for at least 6 months. You can use the protect ranking 8 times but only for one Slam and it is valid for one year. Serena has a top 10 exemption because she finished 2010 in the top 10. She can enter any WTA tournament (except the WTA Championship ) regardless of her ranking until the end of the year.
First post-Wimbledon tournament worth paying attention to.