It is not uncommon for many school districts to be slow in returning certificates of exemption. If it's getting close to the start of the school year, or if you are just feeling uncomfortable, call your school district's homeschool liaison or drop by their office.
You can begin homeschooling your child anytime during the year. You just need to submit an affidavit to your school district. They must accept your affidavit and provide you a certificate of exemption, no matter what time of year it is.
To educate your child at home, you must submit a signed affidavit to your school district. Although the law doesn't dictate the time of year, this is usually done during the summer months. By submitting the affidavit, you are informing the school district that your child will attend a home school and that you assume sole responsibility for your child's education.