When can we to say 'Insha-Allah'? If any one forgets to say 'Insha-Allah' can he say it when he remembers even if it is after 1 day, 1 month or 1 year?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The condition in conditional sentences as well as the exceptions in any other statements need to be clearly stated as part of the sentence itself or the statement i.e. the exception must succeed immediately the thing from which it is excepted. Now, if someone says something and a period of time passes before he or she states the condition of the exception, then anything that he stipulates is nonsense, hence unacceptable. Imam Ibn Qudama says in Al Mughni: ' Ahmad said: the Hadith of the Prophet (Sallallahu Alaihi wa Sallam) to Abdur Rahman Ibn Samurah , 'If you swear to do something and find out that something else is better expiate for your oath and do the good act. Ahmad added: The Prophet (Sallallahu Alaihi wa Sallam) didn't say "make an exception of it," and anyway, if stipulating conditions is lawful in all circumstances, then nobody would break any oath (as he can just add new conditions anytime).
The Shafie and Maliki add yet another condition, that is: to have a firm intention of making an exception, not just seeking, blessing. So, if the person says 'Insha Allah' (if Allah wills) intending to get blessing from Allah, then this exception is unacceptable. The Shafie say that the person has even to intend making an exception before finishing to utter the oath.
And what we mentioned about the obligation of the fact that the exception immediately follows the oath of divorce or the confession is the opinion of the majority of scholars. But some scholars believe that the exception is valid even after a month or a year. But the right opinion is the one of the majority of scholars.
Allah knows best.