Assalaamu alaykum. We know that the rulings concerning the fasting of the sick is that if the sickness is chronic, then the fasting person only needs to feed a poor person for each missed day of Ramadan without making up for the missed days. But if a sickness is temporary, the scholars say that the sick person has to make up for all the missed days after they recover.
1) But is there any mention from scholars or any evidence how long that period of temporary sickness should be? Sometimes a temporary sickness may last quite a long period of time, e.g. 6 or 10 years, and making up so many days accumulated for such a long period could be very difficult and burdensome, and Islam is the religion of ease and common sense, and the prophet, sallallaahu 'alayhi wa sallam, instructed us to make things easy. Please provide evidence.
2) Also, in the case of a long-term temporary sickness for several years in a row, is it permissible for the sick person to give a Fidyah (expiation) every year if they know that it will be difficult to make up so many days?
3) Also, if a sickness is temporary but the sick person does not know after how many years they will recover, is it permissible for them to give a Fidyah every year instead of making up for the fast as they do not know exactly how long the sickness will take and one is afraid that it will be very difficult for them to make up so many days after the recovery.
Please provide evidence, and may Allaah be pleased with you!

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

Your question included many issues, and we will answer them in the following points:

1- The scholars said: If a person is not able to fast due to a chronic disease, then it is enough for him to give a Fidyah. If the disease is expected to be cured, then one must wait to make up for the days, even after the passing of a long period of time without any specification of a certain period of time. They have their evidence; Shaykh Ibn ‘Uthaymeen said in Ash-Sharh Al-Mumti’:

“By following texts and inference, we find that the inability to fast is of two types: a temporary case and a permanent case. The temporary case is that of the person whose disease is expected to be cured; this is the type mentioned in the verse; so the person who is unable to fast waits until their inability is removed and then makes up for the missed days, as Allah says (what means): {an equal number of days [are to be made up].} [Quran 2:184] The permanent case is that of the one who is not expected to be cured, who is the one mentioned in the saying of Allah (which means): {And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day].} [Quran 2:184]”

Shaykh Ibn ‘Uthaymeen also said:

“If a person becomes elderly, it becomes difficult for him to fast; old age is not expected to end because going back to youth is impossible... Hence, if he breaks the fast due to his old age, then it is because he is hopeless of being able to fast. For this reason, he must give a Fidyah. The same applies to someone who breaks the fast due to a chronic disease. Many scholars provide an example of the above with tuberculosis; they say: one is not expected to recover from it; however, this example at the present time does not apply because tuberculosis has become a curable disease, but it can be represented in our present time by cancer. Cancer is an incurable disease [in certain cases it can be removed by removing afflicted organs or killing infected cells, but it can still not be cured], and if a person is sick with cancer and is unable to fast, then his ruling becomes like the ruling of an elderly person who is unable to fast, so he must give a Fidyah for each day.” 

Ash-Sheeraazi said in his book Al-Muhaththab, “If a person is unable to fast due to a disease which he fears will increase and he is expected to be cured, he is not obliged to fast based on the verse, but if he becomes cured from it, he must make up for it, as Allah says (what means): {So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up].} [Quran 2:184]”

2- A person who is unable to fast due to a curable disease; if he recovers after many years, for example, he must make up according to his ability without overburdening himself. It is enough for him to make up at intervals according to his ability, and he is permitted to delay making up as long as his excuse exists, even for years, and he is not obliged to give Fidyah. An-Nawawi says:

“If he has to make up for the month of Ramadhaan or part of it, then if he had a sound reason for delaying making up, such as the continuation of his illness or his travel and the like, then it is permissible for him to delay making up as long as his excuse lasts, even for years, and he is not obliged to give Fidyah due to this delay, even after the passing of many months of Ramadhaan. He is only required to make up because it is permissible to delay fasting Ramadhaan with this excuse, so delaying making up is permissible with greater reason.”

3- Your second and third question are on the same issue, which is about a temporary illness if it continues for several years, and the patient does not know when he will be cured from it, and it may be difficult for him to make up; is it permissible for him to give Fidyah every year? The answer is that giving Fidyah is not valid in this case because the Fidyah is for being in despair of making up, while this person is expecting to be cured, and he may be able to make up for it, even if he is unable to make up for it in a certain period until he becomes able, even after a long time, as we have already mentioned. Ibn Qudaamah when speaking about a person who is expected to be able to make up, said, “If he is expected to become cured, then there is no Fidyah on him; rather, he must wait until he makes up, and he makes up whenever he is able to, as Allah says (what means): {So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up].} [Quran 2:184] He only moves to giving Fidyah when he despairs of being able to make up.”

Allah knows best.