For how long should Qunoot during prayers be said at times of calamity?

Saying Qunoot in the obligatory prayers is prescribed when
there is a reason for that — which is when calamity is befalling the
Muslims. So long as calamity is occurring, then Qunoot should be said for
that, and when the reason is no longer present, then Qunoot should stop,
because the Prophet (blessings and peace of Allaah be upon him) continued to
pray Qunoot against the mushrikeen and to pray for the oppressed Muslims in
Makkah for a month, then he stopped saying Qunoot when the reason for it
stopped with the arrival of those for whom he had prayed Qunoot. This is
indicated by the hadeeth of Abu Hurayrah (may Allaah be pleased with him)
according to which the Prophet (blessings of Allaah be upon him) prayed
Qunoot for a month. After saying Sami’a Allaahu liman hamidah, he said in
his Qunoot: “O Allaah, save al-Waleed ibn al-Waleed, Salamah ibn Hishaam,
‘Ayyaash ibn Abi Rabee’ah and the weak and oppressed believers. O Allaah,
punish Mudar severely and send upon them a famine like that of Yoosuf.” Then
Abu Hurayrah (the Allaah be pleased with him) said that they were saved from
the clutches of the kuffaar and came to Madeenah, so the Messenger
(blessings and peace of Allaah be upon him) stopped praying for them.
Narrated by Muslim (675). 

Ibn al-Qayyim said: He prayed Qunoot at the time of calamity
to offer supplication for some people and to pray against others, then he
stopped when those for whom he had prayed came (to Madeenah) and were saved
from captivity, and those against whom he had prayed became Muslim and came
repenting. So his Qunoot was for a purpose, and when that was achieved he
stopped Qunoot. End quote. 

Zaad al-Ma’aad (1/272) 

If the calamity is still ongoing and has not ceased, such as
the occupation of Iraq for example, or the occupation of Palestine, then
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) stated that in this
case it may be said that Qunoot may be offered for a month and then
stopped. 

And Allaah knows best.