How long does it take for an overbite to worsen?

No time frame: It is difficult to give you a specific time frame since the causes of shifting teeth vary with age, your bite, grinding, and periodontal problems, all of which are unknown in your case. If you have concerns, consult with a local orthodontist who will be better prepared to advise you what to expect in your case.

Gradually: Unrestricted tooth movement may take many years, gradually at first but with increasing rapidity over time. Untreated malocclusions can result in excessive wear to teeth, joint damage, damage to the bone/gum complex that supports teeth, and other problems. See a qualified orthodontic specialist (ask your general dentist for referral) for exam (usually free) and rx options.