What should one do if they swore to abandon something forbidden for a specific period, then forgot the duration, and fears unknowingly committing it?

You must immediately abandon the forbidden act and shun all prohibitions and repent from them, in obedience to Allah and out of fear of His punishment.

It is not permissible to break an oath made to abandon something unlawful. Committing it after swearing an oath makes you liable for the sin of committing the unlawful act and breaking the oath, and an expiation for the oath becomes incumbent upon you.

If you forget the duration for which you swore to abandon the unlawful act, investigate until it becomes clear to you, or act upon what you most likely presume.

If you commit what you swore to abandon out of forgetfulness, there is no expiation upon you according to the Shafi’is, and in one narration from the Hanbalis. However, expiation is incumbent upon you according to the Hanafis and Malikis. The more cautious approach is to make expiation for your oath to avoid scholarly disagreement.