How long do I have to change my plea of guilty for a civil traffic infraction?

Per the FL Rules of Traffic Court: RULE 6.540. TIME FOR AND METHOD OF MAKING MOTIONS; PROCEDURE (a) Time. A motion for new hearing or in arrest of judgment, or both, may be made within 10 days, or such greater time as the official may allow, not to exceed 30 days, after the finding of the official. I hope that I have been helpful in answering your question.

Florida traffic court rules allow you to make a motion for new trial within 10 days or within a maximum of 30 days. You could make a motion to mitigate the sentence within 60 days. Despite these rules, since you paid your citation and did not go to trial, you can file a motion to vacate your plea based on payment of the fine. That motion can be made at any time, and the court has discretion to grant your motion. As a practical matter however, the longer you wait, the less likely it will be that a judge will grant your motion. Some judges or traffic hearing officers have a blanket policy of not granting any such motion after a period of six months and others after one year. However, I have been successful in persuading some judges to grant my motion even when the underlying conviction is five years old. It all depends on your jurisdiction and your judge but to a great degree it depends on which attorney you hire. Make sure that you hire a competent traffic ticket attorney that lives and breathes this stuff on a daily basis to represent you on this matter for your best shot at getting what you want.

You do not ask about whether you have grounds to do this. That would be pretty important.