Folic acid is safe to keep taking for several more months without any time-limit concern, so continuing until January or February is fine on its own. The higher-strength tablet you're on is usually prescribed for a specific reason, such as
diabetes, a higher body weight, or a family history, rather than for routine preconception use.
Next Steps
Check with whoever prescribed the higher strength why it was chosen for you, since that decides whether to continue it as is or switch to the standard preconception strength while you wait. There's no harm in continuing meanwhile.
Health Tips
Please discuss this with your gynaecologist at your next visit so it stays part of your record.