Praise be to Allah.
What is called the adhkaar of morning and afternoon may be done throughout the time that is allocated for any of these adhkaar, which is the beginning and end of the day. It is not stipulated that one should do what many people do, of always reciting these adhkaar all at once in one sitting. The best with regard to adhkaar for which the reward is protection for the one who says them is to hasten to say them before other adhkaar, but here we are talking about dhikr for which no particular number of repetitions is stipulated; rather they are to be said only once.
The numbers of times that adhkaar are to be repeated is a matter of tawqeef [i.e., they can only be known through the religious texts, with no room for ijtihad], and it is not permissible for the one who wants to attain the reward for them to differ in the number of times he says them, otherwise he will be going against what is stated in the religious texts and will be deprived of the reward.
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
It may be concluded from this that it is important to pay attention to the number of times that the text states a dhikr is to be repeated, otherwise it would be possible to suggest that another thirty-three times may be added to the dhikr. Some of the scholars said: If a specific reward is promised for saying a dhikr a certain number of times as narrated in the reports, such as in the case of the dhikr to be recited following the obligatory prayers, but the reciter adds to the stated number, he will not attain that specific reward, because of the possibility that there is a reason behind that number of times and it has some special impact which will be lost by going beyond that number. Fath al-Baari (2/330).
The one who goes beyond that number and regards it as permissible to do more will not be deprived of reward. It may be that the one who adds more does not intend not to comply; rather his intention may simply be to recite more dhikr after complying, not to attain greater reward. This is how al-Haafiz Ibn Hajar interpreted the words of his shaykh al-‘Iraaqi in the words noted above.
With regard to the question, it may be noted with regard to what has been narrated concerning the adhkaar that are to be recited at the two ends of the day that these adhkaar may be divided throughout the entire morning and the entire afternoon, without specifying a particular number to be recited after or before a particular action. Rather this number may be distributed throughout the entire time, according to what is easy for the individual, without adhering to a specific method. But what is best is to recite the adhkaar that is to be repeated a certain number of times in full, in one sitting, for fear of forgetting or making a mistake in the number of times it is recited. But in terms of permissibility, it is permissible and it is not stipulated that the recitation be done consecutively. What we have mentioned is the apparent meaning of the following hadith:
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever says ‘Subhaan Allah wa bi hamdihi (Glory and praise be to Allah)’ one hundred times in a day, his sins will be erased, even if they are like the foam of the sea.” Narrated by al-Bukhaari (6042) and Muslim (2691).
An-Nawawi (may Allah have mercy on him) said:
The general meaning of this hadith indicates that the reward mentioned will be attained by the one who says this dhikr one hundred times in a day, whether he says it consecutively or at different times, or says some of it at the beginning of the day and some of it at the end. But the best is to recite it consecutively at the beginning of the day, so that it might be a protection for him throughout his day. Sharh Muslim (17/17).
Badr ad-Deen al-‘Ayni (may Allah have mercy on him) said:
With regard to the words “in a day”, the word “day” is general in meaning, and it is not known at what time of the day that is, so it is not to be restricted to any specific time.
‘Umdat al-Qaari Sharh Saheeh al-Bukhaari (23/25).
The best is for the one who is reciting dhikr to repeat it consecutively for the number of times mentioned. If that is not easy for him, he may recite it whenever he can, without restriction, doing such-and-such a number after every prayer, and the like. Rather whatever way is easier for him, he may do it.
And Allah knows best.