ما هي كفارة نذر ترك العادة السرية لمدة شهرين في حال عدم الالتزام به؟

It appears the questioner knows the masturbation is forbidden, and they must stop it. The prohibition here is confirmed from two angles: the original Sharia ruling and the vow (nadhir). Scholars have differed regarding the ruling on vowing something that is already obligatory by original Sharia. The majority opinion is that such a vow is not binding, while some hold that it is binding.

Therefore, the questioner must promptly offer sincere repentance and abandon this habit, and pay an expiation for breaking an oath — according to some scholars — as a precautionary measure and to avoid dispute. This expiation is mentioned in the words of Allah the Almighty: "Its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or freeing a slave. But he who finds not [the means to do so] - then a fast of three days. That is the expiation for oaths." (Al-Ma'idah: 89).